KINGX,
As mentioned by nilo, control horns should be installed with the pivot point on or as close to the hinge line of the control surface (at neutral) as possible. One thing that nobody mentioned was the basic rule of servo linkage. You should try to avoid ever having the control horn length (measured from the pivot point to the center of the hinge line) shorter than the servo arm. Doing so will put the servo at a mechanical disadvantage and possibly cause a failure. Having the control horn longer than the servo arm is fine, it increases the mechanical ability of the servo.