Anytime you have a problem like you've described, there is a absolutely reliable way to get the answer in about 5 minutes.
Get out a yardstick and measure the sucker and plug those into:
http://www.geistware.com/rcmodeling/cg_super_calc.htm
It asks for 9 measurements and one option. Tell it 5% for the option and run it. Write down that location. Then tell it 20% for the option and run it again. Write that down, and you have the range for safe CG locations on that exact model you just measured.
If you had a big enough yardstick you could do the same with a C-5a and the sucker would fly safely with a CG in the range that gave.