[Pansy-Mae]

I'm not sure where my data will fit in with your theory, but FWIW I've just measured 4 bra's, two of which I wear frequently, and two which I almost never wear because they really aren't very comfortable.

All four are labelled as 36B.

My actual measurements are 35" underbust and 38.5"/39" at the fullest.
Of the 4 bra's:
The two I do wear measure 26" for one, and 28" for the other, unstretched band size, and both measure 8" from the inner end of the underwire across the largest dimension to the underarm side of the cup.
The two I don't wear, again one is 26" and the other 28" but again both are 8" around the cup.

Weirdly, my right boob is definitely larger/fuller than my left and yet the actual measurements say the opposite!! I assume that is because I am R handed and the muscles are slightly larger on that side so it hold that one more pertly. My actual 'round the boob' measurements are 9.5"L and 9"R.
This should mean that an 8" cup is way to small and yet I don't actually fill them properly.

IMO, bottom line is, its all guesswork and there is no way of predicting.
Oh and BTW, of the 4 bra's mentioned above, the least comfortable of the lot, and the most expensive by far, is a Gossard!