What is the formula for MP Gain when putting an AP point into MP?

Not if you wear int gear.

I thought int gear doesn't affect MP from AP put in, just MP from level?

If otherwise, that's news to me.

Edit: And the reason I say that is because HP washing could be abused if that was possible. Put on int gear, put points into MP, take off int gear, and you can remove less MP than you put in, allowing you to gain MP in an easy way instead of just int gears on level-ups.