Most patients present for reduction looking for a certain post-operative cup size, but as mentioned previously, cup sizes vary by brand, and volume varies by band width. After examining a patient, I can guesstimate the amount required to get a person from their starting point to their end point, and usually I feel like I'm within a hundred grams or so of being accurate.
That being said, the most important aspect is communicating to your surgeon what you want, so he or she can deliver it. I find that asking my patients to bring in an "after" photo from the web or my books that has roughly the outcome the patient wants on roughly the same body size is very effective. Given that visual image, I can really accurately deliver the result the patient wants.