Thank you for the question.
I would suggest that you concentrate on achieving a certain “look”, not a specific cup size, with your breast augmentation surgery.
Unfortunately, no plastic surgeon is able to accurately predict what cup size you will end up wearing after breast augmentation with a certain size implant. As you know, you may end up wearing different bra cup sizes depending on who makes the bra.
I would suggest that you communicate your goals with your plastic surgeon ( I prefer the use of goal pictures) and ask him/her to do his/her best to achieve the results you're looking for. You may want to leave it up to him/her to use a breast implant that he/she feels will do the job based on intraoperative evaluation.
I use intraoperative sizers and place the patient in the upright position to evaluate breast size. Use of these sizers also allow me to select the breast implant profile (low, moderate, moderate plus, high-profile) that would most likely achieve the patient's goals. The patient's goal pictures are hanging on the wall, and allow for direct comparison.
I have found that this system is very helpful in improving the chances of achieving the patient's goals as consistently as possible.
Again, in my opinion, it is best to set your goals and level of satisfaction with the results of surgery, on achieving a certain look/proportionality, not a specific cup size.
I hope this helps.