I have used the formulae below to calculate the C.of G. (measured along the M.A.C. of the wing) for many O/D pattern planes over the last 18+ years, without any problems----the final position usually ends up about 2/3 of the way back between them --hope it helps!

CG = M.A.C./7 + Tail Area x Moment Arm(forward position)
4x Wing Area


CG = M.A.C./6 + 3x (Tail Areax Moment Arm) (Rearward position)
8x Wing Area


Edited to add: The 4x wing area, and 8xwing area,bits, should be appearing below the underlined bits, but they move between typing and sending the post!