I'm struggling with the sizing and am stuck between 450cc and 485cc and my surgeon has suggested Allergan Natrelle Inspira TSFs.

the difference you are contemplating is small. I would go with the 450cc vs the larger one based on long term tissue considerations. However the width of the implant may be more important to you for your final look to get a full or lesser cleavage. 

You should try on sizers to get a good estimate of what the volumes will look like on you.  None of these volumes will make you an E/F.  

Choosing the best sized and shape implant is confusing at best. By the way you are struggling with the difference of about a single shot glass - pretty small difference. Here is my best advice to you and what I recommend to patients: 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 link below

Your best bet is to use sizers, and 3D imaging in order to try to "preview" your result.  It will be very difficult to see a difference of 35ccs.  Best of luck!

There is very little difference between 450 and 485cc. Use sizers and add 30-50cc to achieve the look you like with the sizer. Bigger isn't always better. Discuss your goals with your surgeon as nobody online can understand your ultimate wishes without a lengthy conversation and using photos of breasts you hope to have. Good luck!