The distance from the center of the servo to the hole the pushrod connects to in the servo arm, should measure the same as the control surface to the hole the pushrod conects to in the control horn. This will give you a one to one travel. (i.e. 60 degrees of servo travel should translate to 60 degrees of control surface travel.

If nothing else stick the pushrod farther out on the servo arm, and closer to the control surface on the other end!