I don't find professional measuring works at all. The best thing to do is to go into a shop and on try loads of bras in different sizes. You'll be surprised what a different size can do. I wore 36D for years, and it wasn't until I tried loads on in a determined effort to find something to fit me well, I discovered that 34DD is perfect!!
Another thing - always pay a bit more for a bra - and always try them on before you buy because cup sizes do vary between brands. Buy a good brand. Cheap never works - and for me, neither does M&S or any chain store brand.
Debenhams do quite a large range of branded bras, so it's a good place to pick a real assortment and have a try on session. Charnos are lovely, so pretty - and they fit really well, with ne're a saggy strap!! Good luck!