You forgot that it removes 10% of the defence + 10% attack damage also scales up to 50% at 5 stacks. pair with poison dart at 5 stacks (yes it stacks independently) you have 100% attack damage per second and this is without the enchant poison mastery buff yet. It damage it deals doens't shine on normal mobs, but in dungeons and mvp, it does stack quite well.
Skill set in Ragnarok Rebirth is same as in Ragnarok X, In Ragnarok X,when you become assasin cross,you have the passive skill called Enchant Deadly Poison which turns your weapon to poison element.
The full formula is completely different which currently is unknown. It would take a lot of parameters not just ATK and DEF. This is just the very basic not considering those stats.