Having looked on the internet for a new bra (for a speciifc outfit) I am 3 different sizes according to 3 different calculators! (none of them related to a particular shop). I am a 34C (not a chance!), a 36DD and a 38F 
I can't get to a shop for a fitting before Thursday, which is when I need it for, so can anyone give me any pointers on how to measure properly please?