I honestly don't remember the formula, so to speak. I know that for years I was wearing the wrong size bra... I was wearing B when I was actually a C. The reason I discovered this was a trip to La Senza and a very European sales lady with no personal boundaries that came into my dressing room after I tried on a new bra, spun me around and said with a thick accent, "You not wearing right size bra."
She then brought me the right sized bras and wow! What a difference! All I can say is go check out a really good lingerie store and get fitted properly by someone who knows what they're doing!