You need to set your computer radio to zero-zero for the throttle settings. No trim, center of throws at the center and 100% for the throws.

Now, look at the arm on the servo. If it does not make a 90degree angle to the pushrod, then take the hold down screw out and move the arm to as close to 90degrees as you can. NOW use the radio's centerpoint adjustment to get that arm and that pushrod to make a 90degree angle.

OK.... now..........