If your plane has a wing tube, 1/8" plywood drilled for the wing tube inserted between the wing and fuselage works well. If the cg is off from the wing tube the plywood \ can be drilled for the alignment dowel as well. Then a small 1/8" at the location of the CG will get it done. On my current 33% Champ build I am using a bathroom scale under the mains and a food scale under the tailwheel then calculating the CG from weights and measurements, looks like it will come it at about 40 pounds.