For the missus, I assume. If it was for you, I'd be hella worried.
TBH, it's actually quite easy to measure yourself up. Tape measure under the boobies around the rib cage, and that's the band size in inches. Tape measure over the boobies, and the difference is the cup size. Every inch of delta is up one cup size. You can tweak sizing - rule of thumb is up a band size and down a cup size, or down a band size and up a cup size.
The big problem with bras is that there's not the range of sizes that's really needed as it's cheaper for businesses to make fewer sizes - unfortunately at the detriment od comfort. I do not find it easy to get bras in my size (either a 30DD or a 32D) on the high street.