My limited experience building and flying my own designs, kits, and RTFs has been that balancing on the spar (for a straight spar) or wing tube if there is one is normally very reliable, and will get you very close to the final CG. The spar is typically placed at the thickest part of the air foil, which in most air foils (flat bottom or symmetrical excluding more exotic wings) is close to the center of lift. I have yet to find a plane that balanced on the spar and flew tail heavy.

Brad