If girls undies are anything like boys there is no answer to this question, you can buy the same size (on the label) in three different shops and find they are all different when you try the on. And that's the crunch 'try them on'. Good luck with it Sharon.
I guess I am going to have to find a decent shop with dressing room and try them on. A no no for me are Peacocks, Primarks and Matalan. Their bras don't look like they will last five minutes.
As many have said try them on should sit in the centre of the back and not ride up and the wires should sit comfortably against the breast bone and not dig in under your arm.
I've never bought a bra from anywhere other than m&s.
Ive never been measured there, what I do is take a whole load of bras I like, in different sizes, into the changing room and try them on.
Sometimes the same style in the same size but a different colour fits differently, all down to the machinist who sewed it together I imagine.
My advice is try them all on, never buy without trying them on even if you're buying the same colour and size as last time.