I am a 29-30, and have just ordered some 30 inch bras having read the bra intervention threads.
Compared to my old bras they feel uncomfortably tight, and despite being fairly slim i have spectacular bulges under my armpits. It's definitely not breast tissue, just fat squeezed over.
From what I've read size 30 is about right, I'm a size 8 and measured 29.5. I ordered the bras in a variety of cup sizes, but all 30 inches.
I had planned to send them all back VB and try 32 inches but if they tend to loosen up I might hold onto them. The cups fit beautifully.