Ditto, the main spar is typically located at the thickest part of the wing, which is typically close to where most models balance. But the balance point (BP) really has nothing to do with the main spar. Instead, the balance "range" is determined by the airfoil shape, wing shape, loation and size of the horizontal stabs, etc. The balance point will vary from plane to plane, depending upon design. A typically BP will fall in the 25%-40% mean aerodynamic chord range. This as you can see is quite a broad range, so never assume. Always go with the plans.