Use the longest horns you have at both ends. Set the wires up in a manner that lets you move them in or out on the horns at both ends (servo and rudder). Set your servo travel for maximum travel. Move the lines in or out until the desired rudder deflection is obtained with the servo at maximum travel at high rate. The distance that the line is moved away from the center point, i.e., the center of the bevel on the rudder and the center line of the servo output shaft HAVE TO BE THE SAME. Do what you will with any excess horn length you may have.