The effectiveness of the surface is based on the airfoil section it is connected to. Ailerons, flaps rudders and elevators only act to alter the camber of the airfoil and it is only by looking at how the "new" airfoil works that you can see what the end result is. So there's no way to really say what the effect will be without running some airfoil analysis on the whole wing section. Dr Micheal Selig did this with some of his sections in his wind tunnel research. Similarly you can do it virtually by downloading and purchasing the full license for Profili 2 that comes with Xfoil. Xfoil is an airfoil analysis program that generates lift and drag curves for airfoil coordinates that you can input.

Actually Xfoil is available to download and run for free but it's a complex command line input. Profili 2 provides a nice user interface.

Once you know how the flapped airfoil performs in section you can figure out how much extra lift the portion of the wing with that surface has when the surface deflects.