Tall Paul,
I didn’t find the Palos site very helpful. They had a good diagram for finding the MAC and aerodynamic centre of a wing, but I think they were a bit vague about getting a CG position. I can’t find my print out. I’ll have another look.

What exactly are we trying to find?
The Aerodynamic Centre of a pair of wings combined as a biplane.
That is defined as the point about which their combined pitching moment does not change with Angle of Attack.

Your method – the Palos website method – many unscientific bodges invented by modellers, may get a usable answer some of the time or possibly even often, but they are not science. They luck assisted by experience.

Similarly your Kadet with the optional extra wing proves nothing.
“The plane can fly sans lower wing and no other changes. “
So What? An anecdote with no experimental aerodynamic data is not science.

"Any method other than the graphical computes a c.g. way too far aft for stability for this biplane/monoplane."
Then the methods you used were unreliable. What were they?

I looked up all the textbooks in my study, and the only one which gives a method for finding the MAC and Aero Centre for a biplane combination is the well known “The Design of the Aeroplane” by Darrol Stinton. His method certainly looks convincing as science, and is remarkable similar to the one on the “Designers and Builders” website (the article is BeDesign Number 17) and the one in my book.

Figure 1

Consider this. I want to make a biplane from two identical wings of chord 4 inches. (draw it out). I place them one above the other, with the leading edges of both on the datum mark, and the AC of the combination is 1” aft of datum. By either method.

I now decide to mount the lower wing staggered aft one chord length, 4”. The mean chord is half way between the upper and lower wings. Its quarter chord point is 3” aft of datum.
By the method you advocate you will draw a chord 8” long and place the AC at 2” aft of the datum. That’s an inch too far back. That’s an error of 25% of the wing’s chord, quite a lot.

Suppose you have this pair of wings flying along with a lift force on each (at 25% chord of each?) and the plane meets an upset which increases the AoA very slightly. At that instant the lift on each wing increases, and the increase is at 25% chord., one 1” aft of datum and the other 5” aft of datum. The sum of these two small lift increases will act where? Where does the resultant of two small and almost equal forces act? That is the definition of the Aerodynamic Centre.

If you answer is not 3” (or even 2.9”) aft of the datum, I give up.

A workable CG is all many people want, but I want to get the reasons right too, so that I can justify using it in my book.
Best regards,
Alasdair