Well, not sure what you mean by "perfect mechanical advantage" but as a general rule the control horn (as measured from the center of the hinge line to the pivot point) should be equal to or longer (preferred) than the servo arm. When the CH is longer than the servo arm you have a mechanical advantage at the servo. When the CH is shorter than the servo arm you have a mechanical disadvantage, not good. When they are both equal in length, you have neither. The longest CH that will give you the total desired amount of surface travel is usually the best route. A plane that is set up for precision flying only can afford to have much longer control horns than the same plane that is set up for 3D.