While I understand your point, I think you're seriously overestimating its significance. I agree that it does not account for all mitigation. It does not account for differences in elemental/internal resistances (3-4% variances) which are not significant enough to be considered for most fights (Jarg is a good exception). It also does not account for self heals or Sonic Barrier. Sonic Barrier = 75-100 HPS (depending on normal/hybrid spec) which picks up maybe 8% of damage taken during normal-high damage boss phases.
Sonic/Blade Barrier is responsible for 95 HPS in Defense/Immortal spec, and a full 142 HPS in hybrid spec.
As for internal/elemental damage, consider the following: if I set my internal/elemental resistance to 9% (the same as a Vanguard/PowerTech), my survivability drops by 1.81% on current content. That doesn't sound like a lot, but it actually turns out to be equivalent to almost 91
points of defense rating! (source
) Again, not something you want to just ignore. 28.2% of damage in current content bypasses defensive stats. LagunaD's spreadsheet says nothing about that damage, and thus represents a very incomplete picture of survivability in general.
The best numbers I can find for assassins points to less than 200 HPS including everything (overcharg saber etc). So call it 150-180 depending on endurance stacking. Even if you assume that assassins mitigate roughly equal damage (compared to a hybrid jugg I have my doubts), that's about 13% of their damage taken. I mitigate ~75% of ranged/melee damage (from the spreadsheet and logs).
You can't count Overcharge Saber / Battle Readiness as part of the steady-state self-heal. It's a cooldown, and therefore falls under a different class of theory-crafting. If we filter down to just Telekinetic Throw / Force Lightning and Combat Technique / Dark Charge, we arrive at a base self-heal value of 135 HPS, or 155 HPS with proc heal relic (which is what I use on my shadow).
This is not an insignificant component of my survivability. In fact, if I remove my self-heal entirely, my survivability drops by 11.4% ! That's huge. Not something you can just ignore. And given that endurance actually has a very significant effect on the value of the self-heal (and thus survivability), it is actively wrong for shadows/assassins to make gearing decisions based solely on the three defensive stats weighted against armor.
As a matter of total mitigation that takes me to ~77% and you to ~78% and I'm not factoring in Endure Pain which is up to 90 HPS but probably more like 40 since it's used more as an emergency mini-CD. If you wanted to compare to a Powertech, obviously it's more of an issue, but if they pop Kolto Overload fairly regularly they're pushing 50-70 HPS.
You cannot naively count cooldowns into steady-state mitigation, since they get popped in response to burst phases, not as an active part of the rotation. However, even if you did count Kolto Overload, you're only talking about a *very* small self-heal. 15% of max HP over 10 seconds, once every 3 minutes. That's just 21.6 HPS if it is used on cooldown. Quite paltry.
So the point is that Assassin magical self heals are just not all that special. The differences between classes are not what you seem to think. This makes the spreadsheet a highly valid method of comparing tanks even across classes.
I use the following spreadsheet: https://dl.dropbox.com/u/1679797/SWT...0Stats.numbers
When restricted to current content, it shows all three tanks balanced to within ~1% (when equally geared), it predicts requisite HPS to within <5% (judged by combat logs), and it considers all four attack/damage combinations. I would consider its results to be much more comprehensive and accurate than LagunaD's for the purposes of comparing tanks head-to-head. If all you want is to determine the relative value of *just* your defensive stats, then LagunaD's spreadsheet is excellent.