Take the measurements that're required by this website and plug 'em in. There are 9 measurements and I think you've mentioned most of them. Then plug in a Safety Margin of 10% and click the button. Read out the CG location. Then replace the 10% with 20% and click again. The CG location that gives will pair up with the first one to give you an excellent idea where the safe CG range lies on your design.

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

The value of these established formulas is huge for situations like yours. It gives you absolutely dependable direction as to the safe range for the CG. It allows you to isolate and possibly reject the CG location as a player in your problem.