I have a h9 aresti with flaperons controlled by the 9C as well. For starters I'm not sure if the slider is the best place for the function. You can never really tell how far they are deflected while the plane is airborn. My flaperons are setup on one of the 3 position switches to allow for no flaps, medium flaps and full flaps. As far as setting and testing you might not need as much deflection as you think. On mine I set it up for maximum flaperons to be totally deployed and found out quickly that this was to much. When deployed they severly limited my aileron control. I have maximums now set to about 50% travel but this will no doubt vary for your model. When testing your flaperons just take the plane to an altitude high enough to allow for a mistake or two but low enough to tell how the plane responds. Good luck!