By way of Blessing of Kings (why are all the good blogs about paladins?), here's a nifty macro to tell you if you have enough avoidance, as a paladin tank. How much is "enough"? Basically, enough to ensure that you avoid all crushing blows. If you pump up your dodge, block, and parry, you can push crushing blows entirely off the attack table, which goes a long way towards reducing the spikiness of incoming damage. More predictable damage = easier healing = fewer deaths. So let's see that macro:
/script DEFAULT_CHAT_FRAME:AddMessage("Need 102.4 combined avoidance. Currently at:",0.8,0.8,1)
/script DEFAULT_CHAT_FRAME:AddMessage(GetDodgeChance()+GetBlockChance() + GetParryChance() +5+(GetCombatRatingBonus(CR_DEFENSE_SKILL) + 20)*0.04,1,0.5,0)
A couple caveats: in "(CR_DEFENSE_SKILL) + 20)", the "20" represents your defense from the Anticipation talent. Adjust accordingly. Also, if you want Holy Shield to be taken into account, you have to turn it on first. This macro was made by Raymond of Nazjatar, so all props to him. I love math.