RE: F15 CoG
Sure, the airfoil will have some effect on balance point because different airfoils have different pitching moments, but the small model made of flat sheet balsa, depron, or whatever, will get you close enough that your prototype will fly. Even with a flying-wing design, or tailless delta, you'll learn the location of a basic balance point as well as some idea of the amount of reflex the wing will need.
It's not an exact process, but you can get a model that will fly and, more important, be contollable.