Magical damage taken over time is pretty irrelevant when it comes to stam. Honestly it is. When you talk about magic damage (as you point out) it becomes all about magicalEHP. That is effective hit points against magic. You can theoretically get resist gear, but really the only resist piece is trinket from BH.
Anyways, the point isn't how much magical damage you take, it's how much you take in quick succession. If you're taking small ticks of magical damage all the time, then... whatever, it's largely irrelevant. It will add up to be a lot, but it's nothing your healers are going to worry about. Also you make take a f*** ton of magical damage (see: hour of twilight), but you always are using cooldowns for it and your healers always have a bit of time to heal you back up so... again, sort of irrelevant.
What you really care about is burst damage, the amount of damage you're going to take in say... 5 seconds... that is magical + physical. So for example Zon'ozz with 7 stacks of the buff hitting you for 100k melee + 70 or 80k breath attack right in a row. That's gonna hurt, you might not be able to hit a CD fast enough, and your healers might not catch up... this becomes an even bigger issue if you take a full hit though, so instead of taking 100k melee you're taking 130k melee.
Really... what it comes down to is being full CTC is way too good (I assume you're a shield talk by the way you talk). I honestly don't know jack about druid or DK mechanics, I imagine DK mastery is still good, but basically as a warrior or paladin, it is really important that you reach full CTC. Once you hit that, make sure you maintain it, but ignore mastery/dodge/parry otherwise and just stack stam, because you're right, there are some really strong magical attacks and you will take some big physical and magical attacks in succession and it's REALLY nice to have lots of EHP for those.
So ya... like the theory and math breakdown is kind of irrelevant from the academic standpoint because you need to be adjusting for what the instance requires. Once you have ~391 or so, you can really start focusing on lots of stam b/c you have enough raw ilevel to reach full CTC without many gems.
Also, the exact conversion isn't 1x10 any more, I have no idea what it is, but when I take off my 510 stam trinket fully buffed I lose about 10k hps.
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