What would 450cc look like on me? I'm 5'7", 150lbs, and measured at 12.

It is very difficult to determine the exact size and shape implant you will require to best match your ideal breast image without an examination by a board certified plastic surgeon. Not just any board certified plastic surgeon, but one with many years of frequently performing breast augmentation surgery including different approaches, techniques and implant choices. This is because several measurements not to mention your breast characteristics are needed to determine the optimal implant size to obtain your goals. Without knowing these dimensions it would be difficult to make this determination. For example, the existing base width of your breast will determine, in many cases, the maximal volume per implant profile that you can accommodate. To illustrate; a 100 cc difference may make a significant difference with a narrow base width breast, but much less of a difference if you have a wide chest wall and wide breast “foot print”. Therefore, just because your friend may have a great result with let’s say a 350 cc implant to make her go from a “A” cup to a “C” cup size does not mean that you will have the same result with the same size implant. . Further simply placing implants in a bra to determine the size best for you is not always accurate as the bra often distorts the size, is dependent on the pressure the bra places plus the implant is outside your breast and not under it among other variables. Computer software morphing programs that automatically determine the best implant size can be helpful in some but not all cases (e.g. doesn’t work well in my experience with existing implants, sagging or asymmetric breasts). Using “want to be” photos however are useful if simply provided to the surgeon as I will further explain in the below link.

Thank you for the question. Although useful as rough guidelines and as communication tools, all the modalities currently used to predict what breast implant size/profile will best achieve a patient's goals with breast augmentation are not very accurate, in my opinion. The use of sizers placed beneath a patient's bra, goal pictures, computer imaging technology… are all useful, but not necessarily as accurate as one would hope.
In my opinion, nothing will replace careful verbal preoperative communication with your plastic surgeon, preferably in front of a full-length mirror along with the use of as many "communication" tools” listed above.
In my practice, I use all of the above modalities and then use intraoperative temporary sizers to help determine the best breast implant size/profile to achieve a patient's specific goals as closely as possible. For this reason, I think it is helpful to have the entire range of breast implant sizes/profiles available in the operating room.
Achieving realistic expectations prior to proceeding to the operating room is also an important part of the preoperative communication process. Patients should be aware that the results of their breast augmentation will not necessarily match exactly what they are visualizing with anyone of the above-mentioned communication tools.
Given your questions and concerns, and given that your surgery is coming up soon, I would suggest that you schedule additional time to spend with your plastic surgeon. This additional time spent will be helpful in the communication process and in alleviating some of your anxiety.In my practice, the use of photographs of “goal” pictures (and breasts that are too big or too small) is very helpful. For example, I have found that the use of words such as “natural” or "C cup" etc means different things to different people and therefore prove unhelpful.
Also, as you know, cup size varies depending on him who makes the bra; therefore, discussing desired cup size may also be inaccurate. Again, the use of computer imaging has been very helpful during the communication process, in our practice.Best wishes; hopefully you will be very pleased with the outcome of the planned procedure.

Congratulations on getting the process started!  In my practice I use three techniques to help patients settle on a size.  First is to ask patients to bring in goal photos.  In this way I am asking them to focus more on the "look" they are trying to achieve and to think less about cc's or cup sizes.  Second, as you have already tried, I use external sizers to allow the patient to see themselves with different volumes.  Third, I use 3D imaging which shows you different implant styles and shapes and the 3D rendering of your postoperative expectation.  Using these techniques I have found success in narrowing down a patient's desired implant size to within 25cc of a particular number.  Certainly discuss this with your surgeon and seek out several opinions until you feel comfortable moving forward.  Good luck!