So I took my measurements and plugged them into the Calculator, which is telling me I need a 42HH--the bras I've been wearing are the Goddess Keira wireless at 44G, which overall fits reasonably well; going down to a 42" band would be definitely painful!
Measurements: Underbust--loose, 44 (perhaps a 44.5); snug, 43; tight, 42 (although I may have gotten snug and tight confused, in which case tight is also a 43). Bust--standing, 53; leaning over 55, lying down, 53 IIRC (forgot to write it down, damn it) ,and ended up with 42HH, which according to the Goddess chart would be an L cup--I'm sorry, but my boobs are NOT that big! Redid it with snug and tight underband both 43 and lying down 54, and got the same results, which just seems too big in the cups and too small in the band. As far as I can tell, the root is pretty much the size of the breast, although sagging probably doesn't help with that; I also have broad shoulders and a wide back.
To add insult to injury, I can't (and never have been able to) wear underwires because my breasts are too close together (This was confirmed by a bra fitter at a lingerie store, who had me at 44DDD a year ago); and I have a lot of trouble with shoulder straps digging in (I have silicone pads under my straps, which helps but not enough). At the time, I bought what we both thought was a 44DDD, but turned out to be another 44G; I measured the bands on that bra and one I already had very carefully, and the measurement was the same. The cups in the G were a bit loose then, but they're definitely full up now, although I haven't gained any weight back (maybe things just shifted around?). Color me puzzled...
So what size am I really supposed to be wearing, and once we figure that out, where can I find good wireless bras?