Assign the channel to the 3 position switch.

Set the flaps up so that your servo is centered, the switch is in the center and the flaps are where you want that first click of flaps to be.

Then set the one end point for full up without binding and the other end point for full down and no binding. I would start with very low end points before you power it up and increase them till you get them where you want vice starting with 100, that way if they bind you dont break anything.