CG at 25% MAC of which wing???

On a bi plane how do you know where to calculate the CG?

Try looking up "biplane mean aerodynamic chord" at http://naca.larc.nasa.gov/ . I tried, but the server is broke now. I looked in some old books, but couldn't find anything.

The way itis done on full size biplanes is fairly simple. Sight down on the combined wing planform (from above, of course). Treat the total planform as one wing, and assume a 25% chord balance point. For a staggered wing such as on my 1938 ocke-Wulf 44J, the balance point actually came 4" in front of the leading edge of the bottom wing! Wing incidence or dihedral does not affect the balance point.

The wings affect each other, so if you want make symmetrically flying model you can not move lower wing back. And if you have wings at same place it is the same as monoplane.

If you want really make upper area different or shifted, then you have to calculate MAC and find AC at its 25% for every area separtaely and then make chord between those ACs. The AC of both wing is at that chord proportionally positioned by wing area.