ive not had mine yet so i may not be a massive help, got mine on 19th april.
realistically id say 340 = C, 380s = D but theyll look smaller then they measure. but as you may of noticed everyones different a girl i know had 380s from 32a and ended up an E. i was going for 380cc but now decided ill have 410cc if theyll fit on the day. id always suggest to go with the bigger size, and theres not much difference in them both really xx