Rigby and Peller 'fit' you by looking at your breasts without a bra on.
Thye are expereinced enough to get the size cos they start with 'seeing you' and go from there.
They will always suggest the smalest back size you can comfortably wear, if it is far too tight you have to say so.
The idea is the tighter around you chest (ribs) the better hold it will have for yuor boobs.
I use to work for them, and spend many a saturday now going to dept stores with freinds to 'do' their boobs!!!
Always get your under the bust bit first, dont even bother wondering what cup size you are till then.
If you measure it with a tape measure, where the band of your bra goes, that almost tels you. You may well be a couple inches more, depending on the bra.
eg I measure about 32 round my ribs, but a 34 is plenty tight enough inmost bras.
Nowadays thee are farmore cup sizes availabel for those with smaller backs.
for instance before i worked at R + P, I wore a 36 C,
because that is the CUP size i could fit my boob into, BUT, the back was to big, ie it rose up.
Then i was shown that going down in the back made the band 'fit', and to compensate i had to go up to a dd cup.
Back then, say about 8 yrs ago, things like 30, 32 34, with d or bigger cups didnt exist, but now luckily the bigger bra manus and marks a nd spencers have made more sizes.
So back to fitting, ok, get the back measurment right, and STICK to it, then start getting that and trying different cups, going up or down depending on size of boob.
I do understand that its annoying that you can go for a 34d in one, and it will feel different in another brand, or style, but thats just like clothes. Different manufacturers have their set sizing. Yet, if one goes for the right sixe around the ribs to begin with you CAN get there.
i hope this has helped, becuase, believe me i had NO idea why bras didnt fit before, and its great when you get the jist of it.
Then of course yo get the different styles, the the fun begins.