It's not something I can explain in detail over the web. I use the methods of summing the stability derivatives for the different parts of the airplane. This is the method presented in the book "Airplane Performance Stability and Control" by Perkins and Hage. I'm not trying to sound like a know-it-all here, it just took me a semester college class to put this all together in my head. Trying to explain it to someone else on a noteboard would be painfull all round.

I have the method extracted to a spreadsheet. However, the input variables are listed in coefficient shorthand and would be meaningless unless you know how to do this already. Plus, there are some terms that depend on empirical tables for input values, so you need to have the book as well. So, it wouldn't do too many people any good for me to send them the spreadsheet. However, I could run some numbers for Thunder-man when the time comes. I'm not sure which spreadsheet he refers to in post #8 though.....

For most conventional configuration airplanes - like this one - the horizontal tail is by far the greatest contributor to stability, so some folks have "rules of thumb" that compare the CG loaction the the tail volume cooeficient. THe tail volume is a measure of horizontal stabilizer power. The bigger, higher aspect ratio tail, farther from the wing, will have more stabilizing power, allowing a more aft CG. If anyone wants to present that method, by all means -do so.