it's honestly not rocket science, when you know about bra back sizes.
measure under boobs, your rib cage size. this is what you are.
like all other garments tho, all makes will come up differently, tho not crazily so (except m&s).
when you know what you are under your boobs, get a bra you fancy in that size off the rail, in a variation of cup sizes.
styles vary of course, but if you measure say a 34 and the 34 you pick up is way to tight, then in THAT style, try a 36, and again try different cup sizes in that band.
Fantasie. the cheapest brand we did at R&P and bloody great they are. Another tip. If you find a Fantasie bra in Debenhams you like, do what i do, go on ebay and you can find the same bra sometimes at half the price.
Remember black bras will often fit tighter than the same version in black, it's because of the dye.
hth
pm me anyone for any info.