The measuring system that a lot of department stores use, where they measure your ribcage and add 4 or 5 inches to get your band size, is totally wrong. I don't know why any so called professional bra fitters, notably M&S, still use it when it's so obvious that it doesn't work!
The method that I find works a lot better, is the same as the conventional one, but just take the underbust measurement to be the band size, i.e, just round it to the nearest even number rather than adding several inches. Then the difference between the band size and bust measurement gives the cup size - one cup size per inch.
My underbust and bust are 29" and 36", and I usually wear a 30F. According to my measurements I should need a 30E, which is only one cup size out - not bad compared to the conventional system that puts me at 34B!