As other have stated, personal examination is critical, as each of us may make our measurements of your breast anatomy slightly differently, but have made our surgical decision-making in line with out own particular way of assessing things. Experience plays a role here, as ABPS-certified plastic surgeons who have years of experience doing well over 100 breast augmentations per year have a high degree of expertise, and will usually have a better track record than even a well-trained colleague who only does a few dozen breast augmentations a year.
One thing that has only been obliquely addressed in some of the answers is the choice of profile. Smaller implants in the size range you are considering will have a narrow, high projection (as others have said, 15cc = one tablespoon, an undetectable difference--so let's just assume 300cc implants are the "perfect" size for you).
Mentor smooth round silicone 300cc implants (HIGH PROFILE) have a diameter of 11.1cm, which is pretty narrow. Allergan Natrelle smooth round silicone 300cc HIGH PROFILE implants are even narrower (10.9cm). Slightly smaller implants will be narrower still! If you have a wider breast base dimension than is appropriate for these implants, you will have an unnaturally wide and unattractive cleavage and a "fake" Baywatch coconut look; perhaps moderate plus profile implants may be a better choice.
You are petite, however, so this may indeed be the profile for you. The point I am trying to make is that you do not choose size and profile based on surgeon's "preference" or "habit" or even that you might have "heard" that HP implants look this way or that--you should choose desired size first, and if that fits your anatomy, then profile is selected to "fit" your breast base dimension so you don't end up with wide gaping "Grand Canyon" cleavage!
This is, of course, why you should select ABPS-certified plastic surgeons who do lots of breast surgery! For more information, click on the web reference link below for my Comprehensive Guide to Breast Augmentation. Best wishes! Dr. Tholen