With the radio turned on, center the ailerons/trim. Then connect the aileron linkages and play with the position/angle of the wheel on the servo output shaft until you get the correct RATIO between up & down. This is where you will want the servo arm. Then you can move the rods in/out on the servo arm or up/dn on the horn until you get the amount of throw you want or close to it. This is not an exact science ya know and for many things close IS good enough. Do spend some time playing with it, you will learn a lot.