ORIGINAL: h-ray-RCA
Looks like the answer is to add the effective chord of all the wings (don't include any overlap) and go from 22%-25% back from the LE of the forwardmost wing.
Steve
20-25% of Mean Aerodynamic Chord. Measure horizontal distance from leading edge of forwardmost wing (usually th top one) to trailing edge of rearmost wing (usually bottom one). Divide by 5 or 4 to get 20 or 25%, then go back this distance from the top wing L.E. (if it is the forward wing). This is a good starting point. If you use 20%, it will be more likely to start out nose heavy, which, as you know, is much better than tail heavy.
On a plane with a lot of stagger, such as a WACO, the balance point may be well over half way back on the top wing's chord. On a plane with little or no stagger, such as a SPAD, the balance point will be around the main spar, usually.