I've been wearing bras that fit (intervention style) for years, and used to go to Rigby and Peller and Bravissimo for their fittings from time to time, buying elsewhere between times.
It's going to be a while before I can get to that sort of bra shop. But I've lost a lot of weight this year (yay!) but my boobs are seriously deflated. If I do the standard dangle forwards method to measure over the boobs, I come out as a 32 G. But that really is dangle IYSWIM. The volume has gone, but the skin hasn't shrunk.
Having hunted for a running bra, I found that in Shock Absorber (which come up tight in the band) I'm a 34C, and measure 36 at widest point when I'm in it (it squishes them in a bit).
So, I was thinking of going with about the 32D/32DD sizing as the start point. Does that sound reasonable?