I knew I needed to be re-fitted recently because when I'd raise my arms the underwire would ride up. I thought I needed a bigger band but no, I needed a bigger cup.
I used that reddit calculator and it put me in a 32-34 DD. I went to Bloomingdale's, which is my favorite place to bra shop, as they give you lots of expert assistance. I was literally in and out of the store within 30 minutes! They put me in the exact same Chantelle demi bra I've been wearing for years, just in one cup size larger, so a 34D. I also picked up a Natori bra that is super comfortable. So beware those calculators are only a starting point! Also the calculators always tend to say I'm a 32 but that band size is wayyyyy uncomfortable. I'm right smack between a 32 and 34 band.
The shape of the cup is highly important and can't be figured out by the calculator. As is where the straps lay, and how wide or narrow the cups are placed, and a zillion other factors. Just went back to Bloomie's yesterday as my daughter needed to be re-fitted. I thought she was also going up a cup size but in her case she needed a larger band, same cup size (which technically a larger cup). We spent a lot more time this time, but she walked out with 4 different brands of bras and all of them worked well. They were more full coverage bras but with seaming that creates shape (no molded cups-- those don't work for her).
I'm no bra expert but my main take-away is that I am fuller on the bottom than on the top, hence why full coverage doesn't work for me, but works for my daughter.
One other tip a fitter told me a long while ago that may help you: a lot of women wear their bra band too high. It's supposed to fit lower, and not ride up in back. If it's riding up, you definitely need a smaller band. And when you clasp it, you want it to ride lower. This helps lift the bust in front.