Question: How much deflection should I use for my Flaperons? Can I overdo it?

Answer: Use 50-65% of the maximim aileron deflection. Any more than that will substantially reduce the effectiveness of your ailerons.

Better Answer: If you overdo the flaperon deflection (i.e. when using the ailerons as Flaps) beyond 50-65% of the maximum aileron deflection you will get little additional flap effect but will substantially reduce the effectiveness of the ailerons (i.e. ability to roll left or roll right).

When over deployed as a flap, the aileron cannot move further down as an aileron. The other aileron can move up so you end up with half the roll effect much like that you would experience with having only one aileron. In addition to the weaker  more sluggish roll effect of a single aileron you will have yaw induced by the differential in deflection between one aileron and the other.