I find it rather astonishing that you could even get a 36 band even halfway across your back if you should be in a 44. I measure 37" underbust, wear a 36 which is snug -- and can fasten a 34 but it's too tight really.
Keep in mind that the advice you see everywhere -- "Measure under your bust then add 4 unless it's a Tuesday on a leap year then add 5 etc etc" -- is pure bullshit. Well, it wasn't, decades ago, when bands didn't stretch.
Now you just measure your underbust, snugly, and go with the nearest even number.
We've all struggled with this, I'm sure. When I was first trying to figure this out (when I had smaller boobs) -- if I added 5" to the band, then subtract that from the bust measurement, I'd have had a negative cup size. Well, that's just ridiculous. I had no problem filling out a 36B. But there's no such thing as a 42AAAminus!
Still, even until last year, I was doing the wrong thing. I was wearing 40B bras, which seemed to fit okay, until I found... the Bra Bloggers... bless them, whatever Almighty there is!
If your band rides up in back, it's too big. Period. If your straps fall off your shoulders, too. The main bra mis-fitting problem women have is a combination of too-large band, and too-small cup. You go up one cup size per band size you go down, and vice-versa.
So... I went shopping. Sure enough. Tried 38C, some fit, some didn't. Tried 36D. Hallelujah! Most of those I tried were a good fit. My new bras stay put, my breasts don't fall out the top or try to sneak out past the underwires, and the straps don't fall down. And although it's purely psychological (40B has exactly the same volume in the cups as 36D) it's quite a mental boost to know that I wear a (fanfare!) D-cup!
Onward to the F-G range!