Yeah I'm going to play the "screenshots or it didn't happen" card here. You're in primarily blues, and halfway through Honored with Violet Eye. I find it somewhat unlikely that you've done any serious tanking. So, while your theories can and usually do work well in 5-mans and early Karazhan, they don't hold up in 25-man content.
The math is really easy to work out, and even easier with Aergis' tank simulator which I believe is linked in the Advanced Training forum somewhere.
Every tank's first priority is survival, no matter what your preferences are towards avoidance or mitigation. More directly, your goal is to survive as long as possible; postponing your death, if you will. There are two basic ends to this. Either your healers run out of mana, or you take more damage than they can heal through. Thus, there are two issues to resolve.
Issue #1: Taking more damage than your healers can heal through. No matter what, unless you have 100% miss, dodge, and parry, the worst case scenario WILL occur at some point. Most (all?) bosses are capable of some pretty crazy burst damage, and if you can't survive it without avoidance, you're basically playing russian roulette with 24 peoples' lives. The way you survive the worst case scenario is by having enough hit points, armor, and block value to reduce it down to a manageable level. While adding more avoidance can certainly make the worst case scenario less likely, there isn't enough avoidance available to make it impossible or even unlikely to the point that it can be ignored. This is why we say "If I'm dying to spike damage, I need more HP/Mitigation."
Issue #2: Healers running out of mana. This is where avoidance comes into play. Due to the nature of endgame healing, if you take any amount of damage, you're
going to get healed by everyone in the raid assigned to heal you. That means if you've got 6 people healing you and take 1000 damage, you're probably going to eat about 10 times that amount in healing. However, if you dodge the attack, your healers (well, the good ones) will cancel their heals, and you save somewhere around 5000 raid mana. This is why we say "If I'm dying to healers going OOM, I need more avoidance."
However, it's important to recognize that healer mana can be influenced by many factors outside of what the tank controls themselves. While there are several options to increase the longevity of your healers' mana pool,
there is nothing anyone else can do to help you survive random spike damage. There is no awesome buff anyone can give you to make you survive an unexpected 20k burst in under a second if you've only got 14k hit points. That means the tank's responsibility first and foremost is to make sure they can survive the spike damage, which is why we say that hit points, armor, and block value take precedence. If you're dying while your healers have full mana, you need more HP and mitigation. Once you've gotten to the point where you can survive the worst case scenario (preferably with a little wiggle room), then you can start worrying about your healers' mana pools.
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Me, LOL!
