Lets get some definitions/acronyms going:
[B]ARM[/B]: Your Armor [B]HP[/B]: Your total hit points [B]MSS[/B]: Your chance to be missed [B]DDG[/B]: Your chance to dodge [B]MIT[/B] = .12 + (.88)(ARM)/(ARM+16635): The amount of damage that you will mitigate off of any standard physical hit against a lvl83 boss mob, assuming 3/3 PotP. I.e. 30894 armor -> 65% mitigation from armor + 12% PotP = 69.2% [B]AVD[/B]: DDG+MSS: The % of incoming hits that you will completely avoid. I.e. 3% miss and 40% dodge = 43% avoidance [B]EFF[/B] = AVD+MIT(1-AVD). The general efficiency of each HP you have. [B]TTL [/B]= HP/(1-EFF). The average amount of physical damage you can survive without a heal.
Pre WotLK/3.0 armor had diminishing returns on armor->MIT, which lead to linear returns on armor->TTL, while AVD had linear returns on agi/defense/dodge->AVD which lead to exponential'ish returns on agi/defense/dodge->TTL. Thus it was demonstratable that the correct method to maximize TTL was to stack AVD. However in practice many had a HP threshold that they determined was sufficient for a given boss, after which they stacked AVD.
WotLK/3.0 has introduced diminishing returns on agility/defense/dodge->AVD such that agility/defense/dodge->TTL is linear much the same as armor->TTL had been linear for some time. This has lead to HP/raw stamina being the king statistic for maximizing TTL.
Is TTL the correct statistic to optimize?
Some would answer yes, many would answer no. Correct me if I'm wrong but those who answer no would point to EFF as that statistic to optimize, once a proper threshold has been reached. And it would be the one that I would pick also if I were in a model where I was spending all my time between 1%-99% health. But the truth is that we spend a great amount of time at 100%, and a large amount of healing is lost as overheal.
How to model/account for this?
30k unmitigated physical damage every 2.5s
Healed to full every 5s
Model Avoidance as Mitigation (i.e. 70% MIT and 40% AVD = .82 MIT)
Option A: +1k HP
Option B: +1% AVD
Every 2.5+5x seconds tank A/B takes 5400/5310 damage and is 11.85/11.62 seconds from death.
Every 5+5x seconds tank A/B takes 5400/5310 damage, is healed to full, and is 14.35/14.12 seconds from death.
Every 5s Tank A requires 180 more healing, however whenever healed to full gains 1000 HP over tank B. If the tanks are healed to full over 5s for a 5min fight, Tank A requires 180*60=10,800 more healing than Tank B but gains 60,000 HP that Tank B lost due to overheal.
At every point it appears as if Option A is the better choice. At this gear level/DR is 1k HP a valid comparison to 1% AVD? The model is very rough...