Good luck figuring it out.
How does the body conditioning scale work? I should probably check my gluttons!
I know Babs weighs 5lbs.
It's the feel of the keel and in relation to the muscles there, and what shape the muscle itself takes: concave, flat, slightly round, pretty round. I think the vet rated Butters a 2 when she saw her. Popcorn was a 4. For dogs and cats they go by a 1-5 or a 1-9 for more subtle variations.
https://poultrydvm.com/featured-infographic/chicken-body-condition-score-chart