Would 400cc HP or 425cc HP be best? (Photo)

Thank you for the question and pictures. Based on your photographs, I think that you are starting at a good place and should have a very nice outcome with breast augmentation surgery.  Given that your surgery is coming up soon, I would suggest that you spend more time with your plastic surgeon communicating your goals carefully.
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. In bra sizers can be useful communication tools also. For example, I have found that the use of words such as “natural” or "full C range" etc 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. The use of computer imaging may be very helpful 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 ( after the use of temporary intraoperative sizers).
I hope this, and the attached link (dedicated to breast augmentation surgery concerns), helps. Best wishes for an outcome that you will be very pleased with.

Go with your instinct! If the 450 was too big then I would recommend either a 425 or the other option the 400. There's no way to tell what cup exactly you will be, but if it looked very good then I would suggest it will give you a big improvement. As far as getting to the details of sizing in my practice, this is how we do it:1) My philosophy includes guiding patients to explore reasonable options for breast implant size, usually two to three similar sizes, which allows the patient to visualize how different volumes affect their breast cup. 2) The best guide for a final breast implant size recommendation is derived using measurements from the patient’s own breasts and chest. 3) When measurements are considered then options for sizes can be presented. 4) I use the Mentor Volume Sizing System of breast sizers to demonstrate how these different volumes will look on a patient’s body. 5) This methods offers good reliability and is simple to use, but is not perfect.Your PS is now the best resource. Best,Dr. ALDO.

Very good sizing systems available to allow you to appreciate the difference between the two sizes you mention. In general most women after breast augmentation feel they might have gone a bit larger. In your particular case, with that information mine, I would recommend 425.