Tall Paul,
Thanks for the tips.
I'll try to insert my diagram when I have more time, though yours was probably better anyway.

I see this wing pair as an extremely staggered biplane with a tail, let's say a tail 15% of the total wing area with its c/4 15" aft of the datum, the LE of the leading wing.

Using your method I believe that the CG will come out a bit far aft. Using mine probably a little forward, but that's the safe side. That is, unless you make another mistake to compensate.

Interesting web page you took me to.
On it, item number 4 locates the AC of the mean chord of the pair of wings just the way I do, I noticed. He has located the LE of the mean chord by proportioning the stagger. He then gets pretty mixed up between Centre of Pressure and Centre of Gravity. Ugly.
(The C of P runs from infinitely far behind a cambered section at zero lift, forward almost to the c/4 (say 27% or 30%) just before the stall, and then usually aft again when the wing stalls)

Dinner time now, must go,
Regards
Alasdair