My magplar healer is not BiS - heck, she's a Bosmer and does not wear/farm SPC. That said, she runs 1700 regen but is very diligent about using EleDrain and using Channel Focus. Without Channel Focus, she can get into a bit of sustain probs if solo sweeping a WB. If with a group, heavy resto attacks can regen alot but, with Channel Focus, I don't really have to do that.

Use your own judgement. I think it is just a matter of evaluating the trade offs. I dropped my char's regen down to 1700 in order to add more spell power and that seems to work fine. You can always swap out the glyphs in your jewelry and try diff combos to see what works best for you. Oh, and switching out mundus stones to try different ones is not hard either.

i keep it at 35k magick
18k health
2000 recovery
2400 spell damage

this is bosmer warden healer
spellpower and master architect.

I am able to play all my 5 magicka chars as a healer (full SPC + Worm) in 4-man-vet-stuff. I usually use stat food (no reg food) and can get around 1700 -1800 magicka reg unbuffed (depends on race) and never thought I would need more. I am able to switch to a healer just by clicking a hotkey for Dressing Room, so I don't even change my mundus. The Atronach would give me even more magicka reg, but I rarely change it. I will change my CPs (much more into "Blessed") and my mundus just for certain DLC 4-man-instances.

You want ~17k health and enough Magicka recovery to keep you from running dry. Divines + Atronach + LA passives will usually get you pretty close.

Beyond that you can adjust accordingly: Higher magicka pool, more spell damage, etc.

If you literally can't get your bar to drop past 50% ever, you can consider backing off the recovery and placing things elsewhere, either by set or directly (spelll damage).

The only thing Spell Damage, by itself, will affect is Light and Heavy attacks. Beyond that, it will improve your heals/damage at the same 10.5 (Magicka) to 1 (Spell Damage) ratio everything else does.

Buffs are bonus material, once you are more comfortable with your build. Your main goal as heals is to keep people alive while staying alive yourself. If your build does that, you're good.

If (sometimes) not, or if it feels a little too easy, consider some adjustments that will give bigger heals, more damage, and the aforementioned buff benefits.

Something else to consider: As a templar, if you're parked in Channeled Focus, it gives you an extra effective 480 Magicka regen up to 18 seconds, and extra mitigation for up to 26. So if you're doing that, you're damn near at 4k regen, which is way more than you need.

How much magicka regen you want to have depends on what you want to do and how much cost reduction you have. I personally run no cost reduction apart from being Breton and wearing 7 pieces light armour, with that I have a bit over 3k regen for trials wearing worm/mending + SPC, for dungeons you can go far lower though. In my opinion you as a healer should be able to cast one ability per second without running dry in order to keep up all buffs and pump out the maximum amount of synergies. There are also fights that don't allow you to do a heavy attack a few minutes, so you need to be able to sustain you healing without it or your group is dead and more ideally still keep up buffs, throw synergies and purge while doing so. If you don't want to do active damage your self there is really no downside to running 3k regen. Of course your heals don't hit as hard, but it's still far more then you need. You can even go as far as running 3,5k+ regen and still have enough healing for 90% of veteran trials.

Edit: I forgot to mention that this is for Templar, on my Sorc I go a bit lower (2,8k) on my Warden a bit higher (3,3k).

There is no right answer for healers, really, as long as you get your job done.

The other healer in my guild runs with 1.3k recovery. I run with 2.4k fully buffed.

I tried running that low and absolutely hated it. He tried running that high and hated it too.

But unlike DPSing, we can both achieve roughly the same result with completely different approaches.