Yes, bra sizing is confusing. As Eileen says men’s shoulders are wider. And because of that the upper torso spacing will be different than a woman’s. So your bra cups don’t necessarily center over the moobs. And your bra band material or type of bra will contribute to it’s elasticity. A sports bra is tighter to hold the girls securely.
The best approach is to visit a local lingerie boutique for a bra fitting. If you use breast forms bring them with you. Lane Bryant carry’s larger band sizes and is CD friendly. With COvid they aren’t measuring but the dressing rooms are open to try on a bra. The under band measurement offers a starting spot. Because of the larger shoulders you may need to go up in band size.
The overbust measurement may be off if your bra doesn’t fit right. So subtracting the underbust figure from the overbust number could mislead you. As you’re aware a 3 inch difference is a C Cup, 4 for D etc. But the cup design is based on volume and sister sizes enter the picture. If you increase the band size the cup size decreases.
Even more frustrating is finding a large band with a small cup size. Some styles start with a B cup, others a C. If you don’t use a breast form a pull over style might work or a sports bra style but again, you might need to size up the band. So be sure to try on the bra, adjust the straps, and don’t buy if the bra is uncomfortable.