Here's a diagram of my FTC-1 water rocket. The indicated CG is set to the computed centroid and the CP is set to 'Barrowman'. The actual CG is half an inch from the centroid. In order for this rocket to backslide, the CG needs to be between the centroid and the CP, so I'll add some weight the the end of the rear transition section. I'll shoot for a point approximately half way in between.