Rough guideline
28 back = size 6, 30 = size 8, 32 = size 10, 34 = size 12 etc
Its definitely not exact, but women tend to be in proportion so its useful if you are questioning what back size you should be. (Cup size can't be related in any way to dress sizes, though retailers try to do this because, well, they are thick).
Which is why it was ridiculous in the first place that until fairly recently you couldn't get anything smaller than a 32. If bra vanity sizing was going on, I would have fitted a 32 15-20 years ago, but I've never fitted a 32 properly. Its always ridden up the back.
The issue isn't vanity sizes. Its the fitters.
When I've been measured in the past its depended on where I've been measured, the training of the staff and what stock they have which affects what size I've been told. There are a lot of places, I've actually laughed when I've been told a size. They have given me a 'best fit' size based on the nearest stock they have in the shop rather than what size I actually am. I've found it quite depressing and upsetting at times, that they had no interest in my comfort or trying to get in stock for women who needed similar sizes. The demand has always been there, its just that retailers have been incredibly slow in fulfilling it properly. Even now, you go into retailers who cater for teenagers (who you would expect to be small sizes and stock dress sizes 4-10) and you won't find a 28 or 30 back anywhere in their lingerie section, even with small cup sizes. I await the day that the likes of Topshop acknowledge the existence of the 30 back with baited breath...
The best example was one occasion I was told I was a 32C by a fitter. So to prove the point that she was utterly incompetent and trying to sell rather than give me a good service, I tried one on. It was hilarious and clearly at least 3 cup sizes too small, and it was up round my neck. I think I gave her a bit of a mouthful about just telling people you don't stock a size.
It does have to be said that fitters are getting a lot better as 28s and 30s become more commonplace and training improves. It was only after a good fitter told me that I was a 28/30 that I even started hunting them down which initially was a total nightmare as I didn't know they exist (pre-bravissimo). That lady was a godsend.