I think it's like this:
dmg*crit + 10+lvl*2 + dmg*sneak.
Example case: your basic attack damage is 20 and you are lvl 10. You manage to perfome a stab critical from sneaking, your damage should(if I am right) calculate like this:
20*1.5 + 10+ 20(stab lvl 3) + 20*0.5 (sneak attack lvl1)
Note that this is before defense, it would(if I am right) appear like this in the combat log: 70-defense = total damage
