Alasdair, I spent a few hours yesterday looking for "biplane design" on the 'net. Lots of hits but none too useful for this subject.
I found one which uses something similar to the "stagger method" combined with area ratioing... it's lurking somewhere on my computer if I can find it.
Found dit...
http://www.weebeastie.com/hatzcb1/BratzCG.html
However... the example drawing.. sans tail, it's either a canard with a very large foreplane, or a tandem.
As a tandem, each wing lifts equally. The front wing also has to counteract the nose-down pitch of both airfoils, if they're cambered, so it is loaded higher. The center of the lifts is nominally ahead of the c.g. to counter the nose-up lift of the aft wing.
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I see the example as a no-gap tandem/canard, or a 100% stagger biplane.
As a biplane, I would rely on the graphical (unscientific) method to find where the c.g. is.
M.a.c. is nice to know, but it's the c.g that the airplane sees.
As a tandem/canard, I would expect the c.g. to be aft of the trailing edge of the front wing.
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Image posting..
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