In another thread there is a link to a CG Calculator. It is on Giestware's Aircraft Proving Grounds website. Here it is.

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


I've looked it over and I give it a thumbs up. It does however work kinda backwards for what you need, but it will work. Just insert your data, and pick where you think the CG should be. Check the static margin. Follow the instructions for getting the CG adjusted such that the static margin is in the 5% to 15% range. Keep moving the CG around by typing in different locations until your static margin is in the 5% to 15% range. This then will be your calculated CG. You can then use this CG figure to balance your real plane, i.e. balance your plane where the calculator indicated a CG with a static margin of 5% to 15%.