Two questions:
1) How is the backstab damage figured out? It can't be just calculate and multiply since the multiples don't work out right (ex. 142/4 = 35.5). What gets multiplied and what doesn't?
2) I'm going to try to max out backstab damage. Depending on the formula it should be either a Kensai/Thief or an Assasin with Celestial Fury and Crom Faeyr in their off hand and a Skald playing a tune to backstab to. What else would help?
Backstabbing
The back stab multiplies the weapons damage before strength bonus and secondary damage effects are applied. This means that an assassins, or kensai/thief, gets his bonus damage multiplied but a berserker/thief doesn't multiply the berserk bonus damage. This is nothing set in stone though as I have only come to these conclusions by experimenting with the game myself and not asked a developer for the exact formula.