You should be fine with the servo the way it is, and the control horn. Since the servo is mounted in the underside of the wing, your actual horn tavel will be 20 degrees forward, from 0... and 30 degrees aft, from 0. This is really all the motion the ailerons require for flight, and should be settable in your Tx. During normal operation, the horn will never need to go to either "extreme". Be sure to check your throws for final adjustments. 30 degrees up, 20 degrees down aileron.