Tom I don't know what you mean by the real world. I balanced your G202 1/3 scale using this same procedure and it worked perfect. The CG software does not care if the airplane is a 500,000 lb jet or a 19 lb airplane.
I agree that this method for figuring the CG should be perfect. However, when you plug numbers into any formula (spreadsheet,etc) and get numbers that are very different from what you expect, it is time to evaluate from a "real world" perspective. If you check the balance of the plane at its tips and the result is closer to 5", then there is a problem with your formula/meaurements. The batteries in the scale could be bad, the plane might not have been level, the datum line measurements may be wrong, a sudden gust of gravity, whatever.
Checking the CG with your fingers is a reasonable first step prior to performing surgery on your plane or adding 26 oz of lead. If it confirms the CG, you have only lost about 10 min to assemble the plane. If it is markedly different, you may have saved yourself alot of trouble.
Leonard