Thanks for your thoughts strawberrylush. I understand the idea of cup size being based on the difference between bust and underbust measurement, but this is not what I was wanting to know.
Most designers when drafting a pattern do so with a set of measurements in mind, and knowing what these are can give you a bit of a heads up on what alterations you are likely to need.
Eg. big4 patterns are drafted for a B cup and a traditional hourglass figure. Colette draft for a C/D cup and a more curvy shape. Sewaholic draft for an hourglass shape ... you get the idea.
With this in mind, I might pick my size differently for different designers. High bust measurement for some, full bust measurement for others...
So knowing what cup size, in this instance the Eva dress, is drafted for, will dictate if I choose my size based on full or high bust measurement.