All stabs except Backstab use the following formula :
Regular Stab Dmg = 70 * [Skill Power + P.Atk * Soulshot Bonus] / [P.Def] * [CD Multiplier] * [PvP Damage Bonus] * [Gracia Physical Skill Damage Bonus] * [Proximity Bonus Modifier] * [Focus Death Bonus] * [Attribute Bonus] + 70 * [Crit Dmg Constant] * [Crit Dmg P.Atk ] / [P.Def] * [Gracia Physical Skill Damage Bonus]
Backstab Dmg = 70 *[Skill Power + P.Atk ]/[P.Def]*[CD Multiplier]*[Backstab Soulshot Bonus]*[PvP Damage Bonus]*[Gracia Physical Skill Damage Bonus]*[Proximity Bonus Modifier]*[Focus Death Bonus]*[Attribute Bonus] + (70 * [Crit Dmg Constant] * [Crit Dmg P.Atk] / [P.Def] * [Gracia Physical Skill Damage Bonus])
Soulshot Bonus = 104% hence 2,04 modifier (boosts only P.Atk)
Backstab Soulshot Bonus = 50% hence 1,5 modifier (boosts both Skill Power and P.Atk)
CD Multiplier = Death Whisper/Improved Critical and Dance of Fire yield a 35% bonus, whereas PoF/CoV give 20%. AQ/Baium are 15% each, Counter Critical proc is 100% bonus and Critical Wound is 30%. Multiply each bonus with each other then add it to the formula
PvP Damage Bonus = A/S-grade weapons that are SA'd or oe'd to +4 in case of duals give 5% dmg bonus so the modifier is 1,05, PvP Mod is 5%. If u should use both of these bonuses, u multiply them with each other and then insert the final bonus into the equation
Gracia Physical Skill Damage Bonus = This is the dmg bonus that came with CT2, it applies to all physical skills and is aproximately 10.113%
Proximity Bonus Modifier = Hitting from the side or the back gives a generic bonus since Gracia Part 1 ~] Side = +10% , Back = +20%
Focus Death Bonus = Tests showed that the Focus Death description is totally wrong. The real bonuses are the following :
Attribute Bonus = 71.80% or 1,718 for 150 attribute difference. Unfortunately i only had the chance to test Lethal Blow +1 for element, so this could be slightly inaccurate, but its surely between 1,696-1,72
Crit Dmg Constant = 6.1 (this is a number that had to be "invented" in order to calculate critical damage P.Atk related stuff, lots of thnx to shurcooL, he was the first to find this, sure saved me a lot of time)
Crit Dmg P.Atk = Vicious Stance lvl 20 is 609 P.Atk, Critical Power lvl 6 is 384 P.Atk, Angel Slayer CD is 265 P.Atk, etc. Add all the bonuses up and insert the final bonus in the equation