https://www.calculator.net/body-fat-calculator.html (great calculators on that site)
Use an automatic body fat calculation like shown in the link above.
For that to be possible neck and hip need to be added as biometrics.
Add a graph that displays body fat.
Add a graph that displays BMI.
User defined biometrics would be great too, because people for whatever reason want biometrics that are not in cronometer can configure one themselves.