Crits hurt. Your first gearing priority as a tank should be to become uncrittable. The formula: (83*5)+(5/0.04) = 540 Defense Skill. Remember that 540 is the crit cap as in, the cap for mitigating critical strikes. 540 is not a hard defense cap. Defense will still continue to increase your dodge, parry, and block by 0.04%.
The explanation: The chance to land a critical strike is modified by the difference between the attacker’s Weapon Skill and the defender’s Defense Skill. All mobs have a Weapon Skill of Level*5. So a level 83 boss will have a weapon skill of 83 * 5 = 415 Defense Skill.
All mobs also have a base 5% chance to land a critical strike. Recall
that every point in Defense Skill increases your critical strike
mitigation by 0.04%. So in order to negate the 5% crit chance you will need 5 / 0.04 = 125 Defense Skill.
By the power of math, 415 + 125 = 540. You need 540 Defense Skill in order to be uncrittable at level 80.