Because you are using a y-harness on the ailerons, you can't adjust the up seperately from the down. Because the "up" direction for one is the same as the "down" for the other.

If it was elevator, you could set more servo travel in one direction than the other.

Now, if you get rid of the Y-harness, select a flaperon mix, and plug one aileron in to the flap channel (channel 6), then you can adjust the "left" and "right" directions for each servo independantly. "Left" will mean "up" for the left aileron AND "down" for the right aileron and vice versa.

The best plan is to set the EPA values to the same for left and right. Then just move the servo arm one "spline" or notch on both servos, so that the arms are "swept" fowards, towards the LE of the wing. (assumign the servos are on the bottom of the wing. If the servos come out the top, then sweep the arms to the rear). (this is what Chuck and others are suggesting, just trying another wording).