The truism I heard for years, to determine band size, was "take the actual underbust measurement, and then add 4 inches". This is... not helpful.
And I feel you on the shock. I was wearing a (sprung-out, probably closer to 42, because I had learnt to hate bra shopping and put it off for months) 40D before I got a proper fitting and hit 38F. The difference it made was kind of stunning. (To me, I mean.)