Tall Paul,
I could not find that Designers and Builders workshop again either. Lucky I printed out the pages last time.

Your top diagram is the one I was trying to convey, only you have it labelled wrong. The 8" dimension is NOT the true mean chord.

The 4" line in between the top and bottom wings IS the true mean chord.
Its length does NOT depend on the separation between the two wings. As drawn (correctly) the lines are parallel so the distance between them is constant. Even when the two wings have different chord lengths, the mean chord lies half way, or maybe a third of the way, down (a proportion depending on relative areas) so the gap does not affect the mean chord.

How do you get these diagrams on the Forum page? I don't know how to post them. I have them drawn out in a Word file which I could email to you if you like?

Continue thus:- On the top diagram put a small lift force at c/4 on each wing. They represent the additional lift caused by a small AoA increase. Now find the resultant of these two forces. Draw it on. See where it falls. By definition this resultant acts at the Aerodynamic Centre of the wing pair. The aerodynamic Centre is traditionally at the quarter chord point of the Mean Aerodynamic Chord. That doesn't work by your method. Therefore your method is wrong.

By my method the resultant turns out at the c/4 of the mean chord, and the Moment on the wing equals 1/2rho V^2 times total area times my mean chord times the moment coefficient of the section. That works as it should. Your method gives the wrong moment because you use the wrong mean chord.

Don't you see? It doesn't work. It may be useful as a quick dodge to guess a CG, but it is unscientific. You can't use it to calculate anything. If you use it to calculate the Tail Volume Ratio you will get the wrong answer.
Alasdair