Well experienced board-certified plastic surgeons will be able to give you a much better “estimate” in person than with online consultation. Remember, as you communicate your goals with your plastic surgeon, that a discussion of cup size can be confusing and/or a source of miscommunication. In my practice, I prefer the use of goal pictures to help with this communication process. Generally speaking, I suggest patients undergoing breast reduction surgery have enough tissue removed to help alleviate/decrease symptoms they are experiencing, while preserving enough tissue to feel comfortable about the upper and lower body proportionality.
If still in doubt about your plastic surgeon's recommendations, seek consultation with other well experienced board-certified plastic surgeons in your area. This will help you achieve peace of mind prior to surgery.