The method I use to determine the amount of down elevator when deploying flaps is when you first lower the flaps, set the elevator trim tab to the achieve the desired attitude of the plane. Then after landing, note the elevator position with the resulting trim tab setting. Then program the radio so that the down elevator is set to that position when the flaps are lowered.

There a probably better ways to determine the elevator setting, but that's what I do; it's easy and it seems to work well.

DaveB