tclaridge:

The figure I used (3.4") is based on both wings. It's the classic biplane determination of mean chord, which can be used when the upper and lower wing are of the same chord: upper wing chord plus the stagger (horizontal distance between LEs of upper and lower wings) = Mean Aerodynamic Chord. In the Camel's case, the chord of both wings is 10.25", and the stagger is 3.5", yielding an MAC of 13.75". 25% of 13.75 is 3.4

The book spec, 4", is about 29%. There have been at least two postings on this thread where guys found it was unstable at that, and nearly crashed it on the maiden. I think 29% would be ok if this bird didn't have a scale stab/elevator. It's very small, something like only 11% of the wing area if memory serves.

In any event, everything I've ever read on conventional, constant chord, high aspect ratio wings says to start out at 25% and slowly move it aft until you find the sweet spot, which is going to vary from bird to bird and pilot to pilot. At 25%, it's going to be stable, no disputing that. It's also going to have enough elevator authority to hold it slightly tail low at touchdown during a wheel landing, which IMHO is the only way you want to land this bird anyway.

The final point is, if you make a small error either way when going for 25%, it's not going to result in an unstable or uncontrollable airplane. If you make a small mistake in the aft direction going for 29%, you could be in deep do do [:'(] I absolutely could not come up with a way to make mine fit on the CG machine, and had to use the fingertip method, which has some inherent accuracy problems.