You are forgetting about the guerilla talent which give a x2 damage when sneaking (sneaking cost 1AP when it's level 5). So you can multiply your calculation by 2
Also there is a trait i think that increase backstab damage by 20%.
The trait is 20% increase of chance to hit, not damages.
There's a characteristic that makes sneaking as fast than walk/run without sneaking, I didn't tried it but wonder if it has any effects in combats.
