There is no magic formula unfortunately- even for modern bras! I'm generally a 36F (my bust is 42" but my rib is 36") and find that fits me nicely in modern bras. I once came across an early 60's bullet bra in a 36F and the cups were huge on it!
Yesterday while photographing some stock, I noticed one of the bras ( very early 60's bullet) said 34EE so thought, "what the heck" and tried it on. It fit just like a modern 36F so guess what went in my drawer? :D
When I list my bras on C&C, since sizing is so particular to different brands, I measure the bust size range from the smallest hook setting (unstretched) to the largest hook setting (stretched) so people can see if their bust size falls within those numbers. When I say "bust size", I mean measuring around the fullest part of your bust with a measuring tape.
I also give the cup width so you can compare it with your breast width. LOL, I always need to look for a cup about 10"- 10 1/2" across, alot of the vintage bras out there are about 7"- 8" across in the cup.
Then I do the same with the rib measurement as I do with the bust.
I do often come across larger bust size bras, I've got a pile waiting to be listed on C&C but Nicole is right, they mostly went by bust sizes not cup sizes well into the 50's and then the wearer would have just stitched the bra to fit her own bust shape.
I find the What Katy Did bras satin Maitresse bullet bras come up a bit small so you may have to contact them to see what size would be suitable as there is no stretch in them whatsoever.
So what is the difference between cup size and bust size? I thought one used the bust size to calculate the cup size. And why doesn't the rib cage size (band size) correspond with the numbers - eg. 34-36-38?
Lei