Ok, this has been bugging me for awhile

This is the type of chart I've always seen that is supposed to tell you your bra size:

To find out your body size, wrap a measuring tape around your diaphragm while wearing a bra. Add five inches to this measurement to find out your total body size. If it's an uneven number, round up.

Then place a measuring tape around the fullest part of your bust. The difference between this measurement and your total body size decides your cup size. Here's how to calculate.

* If your bust measurement is up to an inch larger than your body size, your cup size is A.

* If your bust measurement is up to 2 inches larger than your body size, you're a B.

* Three inches larger, you're a C.

* Four inches larger is a D.

* Five inches larger is a DD.

* Six inches larger is a DDD.

Now, when I do this to myself it says I should wear a 30B - is this even a real size?

Also what about models, whose measurements are usually 33-23-33 or 34-24-34? If you add about 2 inches to the waist for the rib cage a 33-23 model would wear a bra size 30C and a 34-24 model would wear a 32B? Am I just confused or do these charts just not work?