First of all, it's really hard to measure yourself correctly. Secondly, they fail on the band size, IMO. If the number is even you add 4", if it's odd you add 5". You don't round up on an even number. Also, some companies want you to measure over the bust as well. It depends on the brand.
I would stress getting fitted by someone whose been fitting for awhile, not just some employee who watched the video. You'll have better luck with this if you go to a shop that specializes in bras.
I agree with Aphil, Bali makes great bras if you are a larger cup size. I was wearing Bali when I was a D and DD. They don't work for me now that I'm a C though.

But that's another thread...
As far as fit, a full coverage bra will cover all of the breast tissue, but a demi, balconette, or push-up is meant to have the "girls" peeking up out at the top. Fit depends on style as well as size.
EDIT: Don't worry about how the models look in the pictures!!! Those pictures are to sell bras, not instruct on how they are supposed to fit. A good bra sales woman will help you in the dressing room too. She'll explain how the bras should fit as well as measuring you to find size.