rule of thumb is 25% of the wing cord is the balance point. If the wing is 12" wide then the balance point would be 3" back from the leading edge. This is for a constant cord wing, like a trainer. Flat bottom trainers almost always balance at the wing spar. On a tapered wing, if you use the wing spar you should be close enough to test fly it. If it is a foam wing, you can use the thickest part of the airfoil.