Breast Implants Size for 12cm BWD?

It is very important to communicate your size goals with your surgeon.  In my practice, the use of photographs of “goal” pictures (and breasts that are too big or too small) is very helpful. I have found that the use of words such as “natural” or “C cup” or "fake looking" means different things to different people and therefore prove unhelpful.
Also, as you know, cup size varies depending on who makes the bra; therefore, discussing desired cup  size may also be inaccurate.
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 press 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.
By the way, the most common regret after this operation, is “I wish I was bigger”.
I hope this helps.

If only it were that simple! You provide a lot of useful information in your question, but there is no simple algorithm or formula for choosing implant size. There are many factors to consider, and height, weight, bra size, and base width are just a few. I like to have patients show me what size they want to be, and what they consider attractive and natural looking. Then, on physical examination I take measurements, identify asymmetries, look at the quality of the skin, assess the need for a lift, note the amount of natural breast tissue, and get a sense of overall proportions. Based on the exam and the desired goal of the patient, I decide which profile of implant and what range of volumes would be suitable. In the operating room, I can use temporary sizers to determine which exact size looks the absolute best for that patient. Breast augmentation is not a simple numbers game, and getting a beautiful result involves good decision making as well as good technique. A crucial factor for success is good communication with your surgeon. I recommend consulting with a board certified plastic surgeon in person. Good luck!