<b>Tanks to be Uncrittable</b>
For raids you will need to get and keep your defense at least 540. Every defense skill increases your chance to dodge/parry/block and chance to be missed by 0.04% as well as reducing your chance to be crit by 0.04%. All mobs the same level as you have a 5% chance to crit you. So once you reach 525 defense you’ll be uncrittable by level 80 mobs, and 540 defense will make you uncrittable against level 83 mobs.


<b>Rating Conversions</b>


<b>Diminishing returns on avoidance</b>
First what are Diminishing Returns?
A diminished return is when something gets gradually less effective the more you get of it. Think of it like armor. If you have 0 armor and you then get 1000 armor, you wouldnt get the same % mitigation as you would if you went from 15000 to 16000 mitigation.

The above picture only shows dodge you get from gear and does not factor in base dodge and talents.

To avoid tanks stacking too much avoidance in Wotlk (no pun intended) Blizzard implemented diminishing returns to dodge and parry.
Diminishing returns (DR) are calculated separately, so it’s better to balance dodge and parry instead of stacking one over the other.
There are not DR on base avoidances and avoidances gained from talents (such as anticipation and deflection), however avoidances gained from buffs that increase agility do suffer from DR.


Further reading and credit for the information:
http://maintankadin.failsafedesign.com/viewtopic.php?t=15212&start=0
http://elitistjerks.com/f31/t29453-combat_ratings_level_80_a/