This is really interesting, but I'm still confused. Please bear with me because I've always wanted to know this mathematically.
So if your (Sharky) under number is 37.5, then you would add 5 to get your band size of 42.5. Take that number and subtract it from your biggest number of 45.5. That would be a difference of 3; 3=C.
Is that incorrect?
ETA: Because I really don't understand how my numbers work here. My "under bust" measurement is 35. The biggest was 38 (maybe 39, no more than 40). 35+5 (for an odd number)=40. 38-40=-2. That's certainly not right. Even if it were 40-40=0, it still wouldn't be right. 🙁