Mel try this. Set your transmitter trims to neutral. Then turn on the radio and reciever with the arms unhooked from the servo's. without the linkage hooked to the arm set the arms on the servo at 90 degrees to the case. you now set the control rods to the length to meet with the servo arms. Be sure your control surfaces are centered before adjusting the rods. After all is adjusted so the control surfaces are neutral with the trims neutral. And with 90 degrees on the horns you should have the same up as down.

Having said that if you want differential (more down than up or vise versa) you only have to move the horns during set-up. If when everything is set at neutral instead of having the horns at 90 degrees you set the horn slightly ahead of the shaft of the sevo and then set the control rods and surfaces at neutral you will have differential. It is used mostly for ailerons which work more effectivly with differential. When you see a control throw that says say 1/2" down and 3/4" up or something similar you will know how to set that up. You can also get the same results by bending the control horns (or toque rods in strip ailerons) forwards and backwards to achieve the same result.

Good luck with your project.

Mark Shuman