You'll always want 5% crit reduction against an even level mob, and 5.6% against a world boss. That part never changes. One defense skill reduces chance to crit by 0.04% That means:
The uncrittable point against a world boss for any level is (5 x your level) + 140 defense skill (5.6 / 0.04 = 140)
The uncrittable point against an even level mob for any level is (5 x your level) + 125 defense skill (5.0 / 0.04 = 125)
You'll always need 125 or 140 defense skill from gear. How much defense rating that translates into is what varies.
Defense rating per level, have fun: Combat rating system - WoWWiki - Your guide to the World of Warcraft