[isobasic]

28FF/G :O I wear a 32D. If I dropped my back size down to 28 I'd need an E ...that's 2-3 sizes smaller than it suggests :S

https://www.youtube.com/watch?v=uPwmcA_qmEQ there may be signs that you're wearing an incorrectly fitting bra. To make sure that the cups actually fit and you should swoop and scoop your breast into the cups
here's an example http://www.venusianglow.com/2008/04/how-to-put-on-bra-correctly-and-why-it.html and you only need to watch the first minute to get the idea. If after this you don't bulge over than the cup is correct. If not you may need to go up a cup size. Just like i said measuring is only a starting point.

Thanks! yeah, I usually scoop automaticalyfor maximum cleavage The bra I'm wearing from m&s id 32DD and is ok in the cup but rides up a little at the back, whereas aanother one I have from there in 30E is too big in the cup and way too tight, so I guess it not only varies store to store but style to style :L