mike,
The geistware application uses the formulas that've been in use by the aircraft industry forever. The one most of us use for models is usually employed to find a safe reliable CG range, but can be of use for modifying an existing design like you're doing. In any case, whatever you decide to create, it'll be the best tool for getting a starting CG location for your first flight on the new bird.

http://www.geistware.com/rcmodeling/cg_super_calc.htm