I have found the following formula for calculating CG on a biplane. Based on this Great Planes is within 1/16" of an inch of being correct at 5 1/2". Heck, I could off that much just in doing the measurements.

CG needs to be about 25% of the chord back from the leading edge. If you have a biplane then draw a line connecting the leading edge of the wings and make a mark at the half way point, then from that point move aft 25%.