To center them I place a ruler on the bottom of the wing. leading edge to trailing edge, and have the ailerons rest on it. Turn on the transmitter then the receiver and adjust the server connections so that it stays at this position for center.

Refer to the plane manual for setting the limits of travel. One of the problems I had with my NexStar when I first got it was that the elevator limits were almost at their fullest. That made it extremely manuever (for a trainer) and was almost too much for me when I was training.