OK, I'm on a bra thread roll today! Apologies to anyone who knows all this already but are you all sure you're in the right size? I don't know anyone (from my size 8 teen niece to my size 14 mother) who is bigger than a 34 back.
OK, here goes. Measure under your bust (hold the tape tight) to get your band measurement. Do not add on any inches to this measurement. Then measure across the fullest part of your bust. Each 1" of difference between the two measurements equals a cup size.
So, let?s say you measure 30" underneath and 34" across? That would give you a starting point of 30D.
The best way I've found to test the band size is to put the bra (in your case a 30 back) on back to front. If you can breathe but if feels tight and you can only fit two fingers underneath the wire at the front, then that's the correct back size for you. If it's so tight it's unbearable then go up a size.
Once you've got the back sorted, it's time to get the cup right. The cup will differ much more between brands and styles than the back (IMHO). Turn the bra round the correct way, lean forward and using your hands, scoop all your breast tissue into the cups (make sure you get all the flesh from round your back and under your arms in). The put the shoulder straps up and adjust as necessary (not too tight as it's the back that should be doing most of the supporting work). If for example, you've started with a 30DD, you will probably find that after scooping, you have some bulging over the cup and quadraboob going on! This means you need to go up one or more cup sizes. Keep going up through the cup sizes until all your breast tissue is firmly in place.
I do not know anyone that is genuinely an A cup. This would mean an across bust measurement of only 1 inch more than the under bust measurement.
Shops like M&S etc are useless as they do not stock a good range and combination of back/cup sizes. Their measuring is also appalling and wrong! No one needs to add inches to the under bust measurement any more. What you measure under your bust is the back size you need to wear. Honestly [hgrin]