2. The cup size
Now there’s a relationship established between the size of the back bad and cup size, you need to address the latter.
As Chanetelle points out “The cup size (the letter) makes no sense unless it is teamed with a back band measurement (the number).”
For each ‘letter’, from A onwards, there’s no one specific size of breast. It actually depends on the size of body too. So a 34 B and a 40B would not equate to the same volume of breast.
You should know if you’ve got the cup size right if it covers the entire breast with a smooth ‘uplifted’ appearance – and no bulging over the top. Nor any gaps between the breast and cup, either.