You get back 1/20 the damage, rounded down. So a hit for 200 gives you 10, a hit for 199 gives you 9. The MP gain happens as soon as the attack is calculated, so if you are waiting for the graphic to check your mp, its already too late.
This is Correct.
BTW this skill scucks bad, its like a bad joke to make you feel you have a TP skill, i even played around with TP Siphon then Damnation but its a joke, the only thing you use TP for its to TP Siphon and do melee skills which work well, but i prefer my own spells. Why TP Siphon at all? I dont use Dart but to auto-lock and to finish off last of mobs HP if at all. Now tho, the times this skill does land, its decent Damage and Decent SP BUT big but, it misses ALOT its a joke of a skill.