Oof, yeah I was about 23 and wanted to help my now wife to get some of the correct size, which was an almost impossible ordeal. Wanna hear the story? Fine:

Taking the two measures was the easy part (and doing it again during her period, because of course the size changes during the cycle, anything else would be too easy). Then I read that the cup size is the absolute difference between bust and band measurement no matter the band measurement. Furthermore since the material is elastic, for a good support, the band should be a tad below the measurement*.

So far so good, went to the store and there are only A-D cups everywhere, E if you're lucky. So basically no matter what exact measure they take between the cups, you're ok if you're thin and have small or somewhat big breasts, or you're a bit fuller and have tiny breasts. Everyone else is automatically screwed. If you're lucky enough to fall into those categories you then have to try on so many to sift through different positioning and forms of breasts until you find one that is comfortable. We had to order some all the way from the UK because it wasn't possible to get anything coming near the correct size here.

*women who wore normal cloth bras before and continued wearing the same size have felt that the elastic hasn't made things better necessarily. Can't find the source for that one right now though.