I love the 425 cc but was told they look smaller when under the muscle so I am thinking of going for 450 cc.

I would not be too concerned that 425 to 450cc as this is just under and ounce difference. I try to council my patients to go as small as they can and still be happy. You will have these implants for a very ling time- not just this year but for many to come. You are best served to come out of surgery thrilled with results. The worse thing after surgery is to feel you are now too large. Best of Luck.

Thank you for the question. No, in my opinion, it is not possible to accurately determine how many ccs (if any) a patient would “lose” when undergoing sub muscular breast augmentation surgery. I suggest patients do not make decisions based on statement such as: “you will lose 25 or 50 cc of volume when breast implants are placed under the muscle”. These statements are simply not reliable enough to utilize clinically. 

In regards to breast implant volume: much will depend on your specific anatomic features; for example, a 25 cc difference may be noticeable on a very petite patient but not on a patient with a larger frame. 

Generally speaking, the best online advice I can give to ladies who are considering breast augmentation surgery (regarding breast implant size/profile selection) is:

1. Concentrate on choosing your plastic surgeon carefully. Concentrate on appropriate training, certification, and the ability of the plastic surgeon to achieve the results you are looking for. Ask to see lots of examples of his/her work.  
2. Have a full discussion and communication regarding your desired goals with your plastic surgeon. This communication will be critical in determining breast implant size/type/profile will most likely help achieve your goals. 
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 or D 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, best not to discuss your goals and/or judge the outcome of the procedure performed based on achieving a specific cup size. The use of computer imaging may be very helpful during the communication process. The use of in bra sizers may also be helpful. In other words, use as many “visual aids” as possible during the communication process.  
3. Once you feel you have communicated your goals clearly, allow your plastic surgeon to use his/her years of experience/judgment to choose the breast implant size/profile that will best meet your goals. Again, in my practice, this decision is usually made during surgery. I generally select appropriate breast implant size/profile after the use of temporary intraoperative sizers and viewing the patient's chest in the upright and supine positions.
I hope this (and the attached link, dedicated to breast augmentation surgery concerns) helps. Best wishes.