I simulated mine with 6 fins the length and diameter of the tubes. This is conservative. The CP is one tube diameter in front of the front of the tube fins. (12" from the rear of the body tube on my 3" version.)Rocsim says the CP is 15.0 inches from the rear. Right now with no motor the CG is about 18.0 inches from the rear. If I put in a 5 oz motor I'm sure I'll need to add an ounce or more to the nose.