Bra sizing / fitting question

[justmetoo]

Bra sizing seems to be one of those ever-puzzling questions. One of the things that puzzles me is calculating band size. Some guides suggest as Cheryl just did, but most say that you should add 4 inches to that measurement to get the proper band size. Why an extra 4 inches???

I always wondered about that, too; and have also heard to add 5 to your band size measurement in inches to get your "correct" band size. This article on wikihow says that is outdated (from the 1930's!). Looks like I should try a smaller size...
I guess you can't really beat getting a proper fitting by a professional though. Of course not all of us have the nerve or opportunity to do so, but if you can it's probably the best way to go.

I did a google search and found several other sites all saying the thing about adding 4 or 5 inches is incorrect these days.