Adding some things I found on the wiki:
"Higher skill will increase the damage done by a successful strike, but the final amount is also affected by your backstabbing weapon's damage. The higher the damage the higher the backstab potential. Because of this, it is always a good idea for a Rogue to have the highest possible damage dagger in their primary slot."
Backstab Damage Calculations
Note: This does not appear to be accurate on Blue P99. A low 50s Rogue is capable of backstabbing for over 500.
http://www.project1999.com/forums/sh...1&postcount=27
At level 51, your offense skill is 215, and your backstab skill is 203.
Backstab uses a weapon damage multiplier of: 2 + (Skill * 0.02).
((Offense + (Str up to 200) + (str over 200 / 5)) * WepDmg * Mult) / 100
((215 + 200 + (55 / 5)) * 13 * (2 + (203 * 0.02))) / 100 = 335.6028
If you had, for example, 200 strength:
((215 + 200) * 13 * (2 + (203 * 0.02))) / 100 = 326.93700
Under 200 starts to hurt even more since over 200 has diminishing returns. 150:
((215 + 150) * 13 * (2 + (203 * 0.02))) / 100 = 287.54700
At level 60, your offense skill will be 252, and backstab will be 225. With a 13 damage weapon at max strength:
((252 + 200 + (55 / 5)) * 13 * (2 + (225 * 0.02))) / 100 = 391.23500
With Ragebringer or any other 15 damage weapon:
((252 + 200 + (55 / 5)) * 15 * (2 + (225 * 0.02))) / 100 = 451.42500