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.

And if you want you can swap some mods around to adjust crit as well. When I run Hybrid im at 348 Crit Rating, Lethality 300 Crit Rating and MM 220 Crit Rating. These are roughly the numbers from what i've gathered in mmo-mechanics.