For a paladin tank interested in raid tanking, the overriding question is "Am I uncrushable?" It's usually a lot of work to calculate it, especially if you want to factor in buffs. Luckily, Raymond of Nazjatar has come up with a nifty macro that calculates your current avoidance for you:
/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)
It's very useful when trying to build a gear set when raid tanking. The only thing to note is that in the line:
(GetCombatRatingBonus(CR_DEFENSE_SKILL) + 20)
The + 20 assumes you have 5/5 Anticipation. If you don't, you have to change this line to reflect how much defense you are getting from Anticipation (0 - 20).
(Snagged from the WoW Paladin forums. Stuff like this makes wading through all the Ret/Holy sniping worthwhile.)
Edit: Added some spaces so the macro will wrap properly in Firefox. You may have to remove some spaces if it is over the macro character limit.
Edit: Changed GetCombatRating call to GetCombatRatingBonus call. Shortens the macro slightly.
Edit: Also, you have to activate Holy Shield before using this to get results with Holy Shield. Otherwise you'll look 30-35% worse than you really are.