While band and cup size are the standard measurements when figuring out bra size, there’s another issue to consider when adding in breastforms; how much man-boob do you naturally have?
Example: My first bra was a lace cup 40 CorD, then I switched to padded/push-up 36&38 C&D bras; 38C being what I wear the most often. A lingerie store owner even measured me and said I was a 36, but 36s are some what snug on me. In 2019 I got my first set of breastforms (40C) and I noticed my bras were raised off my sternum. In 2020 I decided to get a smaller size (38C) and I still had the sternum problem. Shortly after getting my 38C forms, I was in need of a strapless bra. I went to the same lingerie that had measured me. I didn’t have my forms with me and she offered me a 40B. My man-boobs filled the cups and left no room for my forms, meaning my natural chest tissue is a B cup. The next week I went to another bra shop (with both pairs of forms) and found a Wacoal strapless 38D bra, the tag said the cups would fit C-DD. Both sets of forms fit great.
Unless you have a slender build with little body fat, you’ll need to take into consideration how much “man-boob” you have when choosing which size breastforms to buy.
Also, recently I began hooking my bras on the widest “eyes” and this adjustment has helped my bras lay on my sternum better.