You don't ever have to guess about CG. There is an accepted set of computations that have been in use in the aeroindustry forever. And you don't even have to do the computations. Just input 9 measurements and click.

There is an online application that does all the hard work for you. And it works for an existing model or something you're designing. Matter of fact, if you're desigining you can change some of your measurements and get an idea how they'll work.

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