RE: COG ?
Something else worth mentioning, is that adding lead (ESPECIALLY in a trainer) is not the end of the world! It is something we try to avoid on precision planes, but "if ya need more weight, ya gotta add it".
My Gee Bee "Y" needed about 3/4 of a POUND of lead in the nose (Metric = .75 Lbometers), but it flies great!