5'8", 136lb, Under Muscle - 330cc vs. 360cc?

t 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.

30ccs is a very tiny measurement.  Your PS is recommending going a little bigger than 300 because when placed under the muscle, you loose some of the size. 

Generally speaking, if you like the look of a 330cc implant, you may need a slightly larger implant to achieve that look at the time of surgery. I recommend trying on several sizes before committing but 360cc sounds like a reasonable option as long as you have enough soft tissue/breast tissue to prevent rippling.  You can also consider the option of silicone implants to minimize rippling or wrinkling.

I use 3D imaging to assist in the sizing process which eliminates any guesswork.

Best wishes,

Dr.Bruno