By the wacky math used for these measurements, I have a negative cup size.

Ribcage 29" + 5" to make and even number 34"
Full bust 32.5"

Difference, -1.5

Yet in UK/European sizing I wear a 30 - 32 B or C depending on the manufacturer.

I don't get it. I smashed my girls into 34A and 34AA bras for years because that is what the sales clerks sold me - based on similar math. Those bras were freaking torture.