Battle mage (aura vs finishing blow)

I dont know if this was brought up before but since body boost makes dark aura 40% damage boost 50% of the time dark aura would only give a 4-5% dmg boost and 30 m.att. How much m.att do you need to make finishing blow your main skill from 120 instead of aura?

Note: Im not grinding at party play areas, the dot dmg is useless for me.