As Full Lethality, back in live, I like my crit to be around 38%. So that coupled with the +12% talent, I end up with roughly 50% chance to crit on my DoTs But imo, 300 Crit Rating should be fine..

Assuming augment, and the +3 crit chance at the bottom of Lethality's tree, that would be roughly the end result of 300 crit rating.

I'm at 37.7ish with I think 305 of crit rating. Running MM I get a total of 36,7 % since I take the 3 points of Explosive engineering for a 31/8/2 spec.