Would you be way off? Probably not.

Since the CG location is really a function of how big the horizontal tail is, how far back it is, how big the wing is, and what the wing chord is, why not use an online application that takes those numbers and spits out where the CG should be? We place the CG to control the pitch stability. The tail and those other things are what controls that stability.

http://www.geistware.com/rcmodeling/cg_super_calc.htm is an online application that does the figuring for you. You just need a yardstick and time to make 9 measurements.