rgno1,

I think by the lack of replies you can see this is a tough question. For control surfaces or flaps, most model designers figure what they need by simply comparing to other existing aircraft. Basically what these do is change the airfoil. So the new airfoil section shape with surface deployed should be tested and measured in a wind tunnel just like for any other airfoil. Perhaps one of the computer airfoil design simulators can do this? There may be someone out there with some empirical thumb rule formulas for this. But so often those types of things aren't good enough to produce meaningful results. For models, build it and try it is usually easiest way to evaluate.

Hope that helps.

Multiflyer