Unfortunately there is a bit of variance between brands and even within the same brand. Experimenting is a necessity IME the back size tends to be more consistent than cup size. The returns policy at brastop is pretty good so you could always order your Bravissimo size and maybe one cup up, and see what feels best?
Did the fitter at Bravissimo talk you through what a good bra looks like? They normally do this so you can get the right size in other places. Personally I wear a cup size smaller in the Deco styles than in almost everything else - it comes up quite big.