Sizing help please!

Hi, I’ve just bought the Fira top/dress but am very confused by the sizing. I love that it comes with different cup sizes, but don't know whether to choose my base size using my bust size or to go backwards from the D cup bust size? Does that even make sense! My upper bust is 34inches and my bust is 38inches, so I’d go for a D cup. However, the finished size for a size 12 with a D is 43.5inches. Should I be looking at a size 4 instead? I’m worried I won’t be able to get it on! But the 12 sounds like it will be massive. Usually you’d use upper bust size as the base size to make sure it fits across the shoulders. Please help!