The reason it's difficult is because you shouldn't be measuring yourself!
In fact, I don't like the idea of measuring at all. In the first place, it usually demands that you measure youself wearing your current bra...which is silly, since your current bra probably doesn't fit. Second, there are a lot of factors that aren't taken into account. It only takes into account your overall measurement...not how it is distributed.
I never measured myself to find my clothing size--just tried stuff on until i found something that fit. Same with bras. Measuring can be an ok starting point, but too many people rely on it as the definitive answer and don't try on (or even have access to) a good enough variety fo sizes. If I relied on measurements, I would be a 32D rather than a 30FF. When I was smaller, I measured out to a 32B when I was really something more like a 30C or 30D.