The servo should rotate at least 45 degrees either way from neutral. So if the length of the servo arm (measured from the center of the servo to the outermost hole) is the same as the distance of the control horn (measured from the hinge line to the closest hole on the arm) you should get exactly the same deflection of the surface (45 degrees). This is generally a good amount of throw for the 3d Foamy planes. In fact, with the epa set to 120, you should get even more than 45 degrees.