Dan, the app you suggest is good when the model is going to have a "regular tail". What that is is anyone's guess. To deal with a model that might have a smaller, or larger, or longer, or shorter tail than "regular", the best app to use is one that also considers the details. It only takes a minute more to measure how far back the tail is and what it's measurements are. This app http://www.geistware.com/rcmodeling/cg_super_calc.htm gives a CG location that takes everything important to the CG location into consideration. And it doesn't take any more effort than the app that assumes your model is "regular".