Men have a rib cage that is near parallel; Women's have a significant taper, giving them a higher waist. The bra band size should be the size, just below your armpits. Breast tissue makes it impossible to measure this accurately, hence the formula of measuring under bust and adding 4 inches. Great; given that most British women wear a band 2 to 4 inches too big and a cup size, or 2, too small. As a genetic male, what hope?
As starting point, I suggest taking your under bust measurement as the band size. An under wired bra should perfectly encompass your boobs. You will instantly know, if it is too small. Too big, and you will see the marks of the wire's fall outside the radius of your boobs. Simple, but bra fitting is an art, not science and there is quite a variance between manufacturers. Here, to set your size, a quality bra, from a top name, is really important; only then will you know if that cheap bra is telling you the truth, or is a load of rubbish.