It depends on the length your horn is set to. Use arm length/horn length=ratio of deflection. If you use a 1.25" arm and a 1.5" horn length then surface deflection degrees = 1.5/1.25* servo rotation.

In this case if the servo rotates full 60 degrees then surface deflects 1.5/1.25*60=50 degrees of deflection.

A longer arm puts more stress on the servo and gears.

TF