The number (36,38, etc.) refers to the circumfrence around the chest in inches. The letter refers to the actual cup size an is not directly & exactly related to the chest circumfrence.
Really, this means that a woman with a large chest (say 42" can have small breasts, while a woman with a small chest (say 32") can be busty for her size.
I assume that you will be getting implants to make up for the volume deficit. Likely, you will need expanders, and the process will take many months. If I am correct, work with your ps, and pay attention to how much fluid he inflates the implant. You can use this information for when you have the permanent implants placed.
In my opinion, the most important thing about implant sizing, is getting the diameter correct. Beyond that, the projection can be adjusted with overfill, or different projection implants.
