It would be neat to see the "A Bra That Fits" formula implemented, since getting an accurate bra size depends on more than just underbust & (bust - underbust); for example, varying levels of projection and shape differences. For reference, the calculator itself can be found at [1], and the process for calculating in [2].
[1] https://www.abrathatfits.org/calculator [2] https://www.reddit.com/r/ABraThatFits/wiki/manual_sizing_gui...