Don- I keep record on all my animals, feed,refuse,shed,weights after defecates,defecates. I know my snakes average weight gains.
I think when I have my first clutch my hold back I might have to do a little experiment and weight snakes, food, and keep records to try to figure out an general average for the retained weight of the food consumed. This will be a lot of work but could be a cool experiment or turn out to be a complet waste of time lol.