A well fitting bra won't have your bb's spilling out anywhere, either under your arms or over the top of the cup (not rally applicable with full coverage bras). Your bb will feel comfortabley held in the cup, with a supported feel too, but not necessarily held up too high (after straps adjusted properly). Shoulder straps should be adjusted so that the 'band' of the bra is level from the bottom of the bra at the front (where the bottom of the underwire would be, if there were underwire) to the clasps at the back. You might need to take a few bras to the change room, and take the time to work on adjusting straps, etc. Also when the adjustable clasps are done up at the back, it shouldnt be too tight, aim for a size where it does up on aboit the middle clasp.

There is the proper way of measuring for sizes, but this could be easier. Did you need me to explain that?