Ratings are different for different levels:
- For a level 60 : 1.5 DR = 1 defense skill
- For a level 70 : 2.36 DR = 1 defense skill
- For a level 80 : 4.92 DR = 1 defense skill
Each point of defense skill above your base defense, decreases your chances to be crit by 0.04%. And, at your level, a mob has a base chance to crit you of 5%. For every level beyond the player, there is an additional 0.2% chance for the mob to critically strike the player. This is why lvl 83 mobs have a 5.6% chance to crit a lvl 80 player, just the same as lvl 73 mobs against lvl 70 players.
From this data, you can infer the defense rating needed to become crit immune at any possible level combination. Keep in mind "boss" level mobs don't have static levels, that means a ZA boss doesn't hit a lvl 80 player like a lvl 63 mob.