Bra sizing and sizes are a nightmare. A couple of years ago, after saving some, well in truth, many pennies, I decided it was time to treat myself. I went down to London to Rigby and Peller for a proper fitting. R+P supply and fit the Queen, so I decided that they must know what they are doing.
Firstly, they do not use a tape measure. They are so experienced that they can tell just by looking at you what size you are. I thought that I had been doing a good job of measuring myself as a 34DD, but the lovely lady fitter shook her head and said that I wasn't. She left me for a few minutes to collect some bras to try on. She helped me put them on, adjusting the straps in seconds and voila, an absolutely perfect fit. My boobs were right where they should be, and all without scaffolding 
As for my actual size? I am a 32 E 
Yes, the bras themselves are expensive, but they are lovely quality.
Well worth saving my pennies for believe me.
As for the hooks, the fitter told me that you start off with the first hooks because as you wash the bra, it will stretch over a period of time.