Actually, that would depend on your CG. If the CG is back far enough you'll need down elevator. If the CG is a little more forward, you'll need up elevator to keep it level. Usually, on a new plane, I'll hit the flapperons in flight to see what it does, then make adjustments from there. speed will also play a part in it. Another way to test it is the yard dart technique. Either way will produce the same result. There is no "one answer fits all" to that question. Depending on the plane, I would program the elevator for level flight at slow speeds. I only use the flapperons on my 3D planes, and they don't fly fast anyways.