in an ideal group setting, the main tank is usually providing about -12k armor on the enemies

in most content (dungeons, trials) the enemies have 18,200 resistance

major + minor breach is approx 9000 armor
then the tank could be running crimson oath which is another approx -3500

those 2 combined is -12.5k armor

then they could stack on another approx -1500 from tremorscale bringing the total -armor to about -14k which leaves the enemy with about 4200 armor that you could easily get depending on sets and skill lines (arcs have no problem hitting about 7000+ pen usually)

if your in an unoptimized PUG, the best you can hope for is major + minor breach as those are fairly commonly sourced

Nyseto wrote: »

I have an add-on that tells me the penetration and I find that with thief mundus I'm almost never at the penetration cap in group pve for pugs that is. Most guides say Thief mundus is best most of the time but it seems to me that only applies if you're in an optimized group?

Thief is the best mundus even for unoptimized groups, because the other stats are much easier to come by. Lover mundus is often an overkill, leading you to waste a lot of stats for nothing. For penetration better sources would be penetration lines from sets such as Tide-Born, any monster set with a penetration 1pc such as Arch Druid or Jerensi. Second choice is to wear a few light pieces to cover for missing penetration, such as a light monster set piece. Crit damage is the second most important stat when it comes to damage dealing, which caps at 125%. For that you can use more medium pieces, use Fighting Finesse (recommended) or Backstabber CPs, but most of your crit damage should come from the groups supports. If you find yourself often lacking crit damage you can also use sets like Sul-Xan or Order's Wrath and maybe even use one or two axes instead of daggers.

For crit chance is the first important stat you wont be able to reach the cap on without seriously sacrificing from elsewhere. The only reliable sources for crit chance is thief mundus, daggers or bow, light armour and sets. If you build into light armour for the crit, you are very likely over capping on the penetration if your tank is bringing sufficient buffs. You will also get less medium armour which is going to cost you crit damage and weapon and spell damage.
You want as many sources of crit chance as possible, without sacrificing the first two stats and that is why thief is always recommended.