you don't add inches to your under chest measurement.
it was me who tried to explain!!
ok, inches were added on to enable the poor fitters, who had not many sizes to get your boob to fit into a cup. Yrs back (and not that many) there were only these sorts of sizes
34, 36, 38, 40 , 42 a, b, c, d
have a look, especially at those cheapo bras in the supplemets.
so
if you had a big boob, and were actually a 34 tho, around your chest, they would add a few inches to that round the chest (under boob ) measurement so as to get to say a 38, or even a 40. this would then ensure a large cup.
BUT
now, you can get 30, 32, 34, 36, 38, 40 etc in all the cups.
i used to be a 36 c, before i woked in rigby and peller. yes, my boob fitted into the cup. Think bigger band (ie under the boob) = wider and biger cup. BUT the band itself was too loose around my back.
36c
34d
32dd/e
i was actually a 32 (fitted my underboob) dd.