First, it is important to know that ZOS has assigned static values to mob mitigation in ESO. Overland enemies (excluding World Bosses) have 9100 mitigation, while Dungeon mobs (excluding Delve and Public Dungeon mobs) have 18200 mitigation. Resistance to a particular damage type increases mitigation by 125 for Overland and 300 for Dungeon mobs.

The formula for calculating the % amount of Mitigation a mob has is: MITIGATION = [(Resistance - Armor Debuff) x (1 - % Penetration) - Flat Penetration] / 500
If we were considering PvP, the bottom number would by a 660 when facing a CP 160+ character.

Let's look at a Dungeon mob without any reduction in mitigation: 18200 / 500 = 36.4% mitigation
100 - 36.4% = 63.6% damage done
So if we attack this mob with a skill that has a tooltip of 10000, 36.4% of that incoming damage will be mitigated, meaning 63.6% of my 10000 will get through: 6360 actual damage done.

Now, in most cases, a PvE Magicka DPS player will be wearing at least 5 pieces of Light Armor. This allows access to the Concentration passive in the Light Armor skill tree, which grants 4884 Spell Penetration. Also, every character gets a free base 100 Spell/Weapon Penetration. So without any real investment into Spell Penetration, we can expect to have 4984 on hand.
(18200 - 4984) / 500 = 26.432% mitigation
100 - 26.432 = 73.568% damage done

We saw that with 0 Spell Penetration we do 63.6% of our potential damage, and with 4984 Spell Penetration we do 73.568% of our potential damage. Now you might think we should SUBTRACT the lower number from the higher to get the difference in damage we do: 73.568 - 63.6 = 9.968. But this is actually WRONG! What we really need to do is DIVIDE them: 73.568 / 63.6 = 1.1567. You can drop the 1 and move the decimal over 2 places to get your % increase: 15.67% damage increase.

Let's confirm this with our 10000 damage attack:
10000 x 0.636 = 6360
10000 x 0.73568 = 7356.8
7356.8 / 6360 = 15.67%

And here is where things get tricky. Since PvE mobs are level 50, that means that every 500 mitigation they have = 1% mitigation. So it would seem that if a mob has 5000 mitigation (10%) and I apply 5000 penetration, that should equal a 10% damage increase. However, this is actually not the case.
Including the added penetration:
(5000 - 5000) / 500 = 0
100 - 0 = 100
Excluding the added penetration:
5000 / 500 = 10
100 - 10 = 90
Dividing the results together:
100 / 90 = 11.11% damage increase

What about if the mob has 15000 mitigation (30%) and I apply 5000 penetration?
Including the added penetration:
(15000 - 5000) / 500 = 20
100 - 20 = 80
Excluding the added penetration:
15000 / 500 = 30
100 - 30 = 70
Dividing the results together:
80 / 70 = 14.28% damage increase

So there is a greater gain from Spell Penetration earlier in the process of penetrating, which diminishes the closer you get to fully penetrating the mob, though never falling below the % of that mob's remaining mitigation. The last 500 penetration applied would be worth about 1% more dps.

This chart represents how much more damage you would do to a PvE mob by adding penetration from an initial starting point (i.e. If you are already are at 75% penetration, you would gain 33.3% more damage by getting to 100% penetration, and would gain 20% more damage if you stopped at 90% penetration)



Although it is true that the benefit of increasing penetration diminishes, even the last 2500 penetration still amounts to a 5.2% damage increase. It therefore is worth trying to get as close to the penetration cap as possible, though the difference between 17700 penetration and 18200 will be rather negligible (1%, and less the closer you get).

Since we want to be as close to the penetration cap as possible, without exceeding it, we need to consider the methods by which we can gain penetration for ourselves as well as how other members of our group can help us gain additional penetration. If we are playing solo, or with group members that are not providing any Spell Penetration buffs for us, it is in our interest to use as many means of gaining penetration as possible ourselves. If we are playing in a well coordinated group that provides many buffs to Spell Penetration, it will be much less important to spec into that stat. Therefore, HOW MUCH PENETRATION YOU NEED IS ENTIRELY SITUATION DEPENDENT!
Questing Oveland? You don't need to bother going over 9100 penetration unless you're going to take on some World Bosses.
Doing vMA? Get your penetration to 12.1k, it's the most you need before applying Ele Drain (which is only useful against casters).
Doing a Trial with a coordinated group? Check how much penetration will be provided for you.You may only need 9-10 points in Spell Erosion to hit the cap, and nothing else.

Here are all the different ways you can gain Spell Penetration:
- 100 Base Penetration
- 4884 Light Armor Passive (Concentration)
- 4196 (7 gold divines); 3577 (4 gold divines); 2752 (0 divines) Lover Mundus Stone
- 2752 Sharpened Weapon Trait (gold)
- 3450 Spinner's Garment 5th piece bonus (gold Light Armor)
- 3010 Roar of Alkosh 5th piece bonus (gold Medium Armor) - 10 sec duration on synergy activation
- 2741 (Infused+Torug's); 2108 (Infused); 1622 Crusher Enchant (gold)
- 5280 Major Breach - One Hand and Shield Ability: Pierce Armor (Morph of Puncture), Destruction Staff Ability: Weakness to Elements+Morphs, Nightblade Assassination Ability: Mark Target+Morphs, Warden Animal Companion Ability: Subterranean Assault (Morph of Scorch - AOE)
- 1320 Minor Breach - Templar Dawn's Wrath Ability: Power of the Light (STAMINA Morph of Backlash); Also available on some poisons, but uptime would not be very high
- 105-5280 Spell Erosion Champion Point Star

Since the Spell Erosion star is the easiest element to fine tune, we should save it for last in our decision making. Also, as with other Champion Point stars, it will give diminishing returns the more points we put in it, so we should probably not go far above 50 points (3960), at least not until the CP cap is around 900.

This chart shows a breakdown of different ways you could reach the Spell Penetration cap on Dungeon mobs (18200) in either solo or group environments. These are just a few hypothetical ways you could put the numbers together. Feel free to apply your own preferred combinations, or discuss with your regular group to determine what best suites your situation.



SOLO (No Breach) - This would be for a situation where you were playing alone, perhaps completing an Undaunted Pledge solo and either didn't have room for Elemental Drain on your bars, or wanted to fully penetrate all mobs instead of just bosses (because you wouldn't be applying Ele Drain to every mob).

SOLO (Breach) - Similar situation as above but you are including Ele Drain on your bar. Or you are playing with other people and Major Breach is the only group penetration buff you are getting.

SOLO (No Sharp) - Similar to the situation just above, but you want to use a different weapon trait than Sharpened.

GROUP (Best Case) - This represents a fully optimized group that is able to have tanks running Alkosh and Infused+Torug's Crusher Enchant with nearly 100% uptime, as well as a Stam Templar keeping Minor Breach up.

GROUP (Consistent) - This represents a less optimized group. You have a tank running Infused+Torug's Crusher, which is pretty easy to keep up 100% on the boss, but you have no Alkosh or Minor Breach.

Since vMA remains the only place in ESO that does not conform to universal Overland/Dungeon mitigation levels, and the previous analysis only tested physical mitigation, this is the full analysis including spell mitigation as well.

Arena 1 BOSS: 9.1k phys; 18.2k mag

Scamp: 18.2k phys; 18.2k mag
Flesh Atro: 18.2k phys, 12.1k mag
Clannfear: 12.1k phys; 12.1k mag
Seducer Knight: 12.1k phys; 12.1k mag
Daederoth: 12.1k phys, 12.1k mag
Bone Collosus: 12.1k phys, 12.1k mag
BOSS Adds: 12.1k phys; 12.1k mag
Seducer Darkbow: 9.1k phys; 18.2k mag
Seducer Darkfire: 9.1k phys; 18.2k mag

In the 2nd Arena couple of the mobs, such as the Dwermer Spiders only have 9.1k mag resist, but all other mobs in vMA have at least 12.1k mag resist.

The general premise seems to be that mobs that use magicka skills (and sometimes magical beings like Lamia Ghosts in Arena 3 or Lurchers in Arena 6, but not Fire Elementals or Lurcher Bosses in Arena 9) are more resistant to magicka, and vice versa with physical, while those that have a mix of both are in the middle. Since most mobs have 12.1k mag resist, speccing for that and then using Ele Drain on casters makes the most sense as a Magicka character. A similar (opposite) strategy could be taken for Stamina characters.

Assuming the other bosses have the same Phys Resist as the linked post...(someone else can double check if they want)

Arena 2 BOSS 12.1k phys; 12.1k mag
Arena 3 BOSS 12.1k phys; 12.1k mag
Arena 4 BOSS 18.2k phys; 12.1k mag
Arena 5 BOSS 12.1k phys; 12.1k mag
Arena 6 BOSS 9.1k phys; 18.2k mag -> (33.5k mag when enraged)
Arena 7 BOSS 18.2k phys; 12.1k mag
Arena 8 BOSS 18.2k phys; 0 mag (14.4% extra mag damage done during disorient phase, if this was a debuff on top of resistances then perhaps applying Ele Drain would make the damage increase higher, it's worth testing) Note: The 2`hand ult should be tested again during the disorient phase. My guess is it was originally tested when the boss was shielded.
Arena 9 BOSS 9.1k phys; 18.2k mag