It really looks like they just add the base stats to get your best girl. And I believe that what gets added from coaching is related to the ratios of base stats of the best girl. I have a couple that ended up with individual stats higher than the best girl. It may be that they matched the sum of the base stat and that I didn't see that since I continued to train the best girl for the days the coaching went on. It looks like you need to have each of the best girl's stats higher than another girl's to be able to coach her. My practice runs a full day, so it may be a while before I can test that.