I was thinking somewhere along the lines of (X + X.XX) *Y% where Y is the amount of mana missing.
For Example (This won't be balanced at all, but this is just a representation)
With 30 int and 50% mana:
(100+ 2.00*30)* 50%
80 damage.
I like where this is going.Besides the OP Presentation.This should definently be implemented for Runeblades