Okay. No matter what "I" measure those never work out for me.
When I measure I should be in a H or something. When I put on a DDD bra in the larger size around for me it gaps all over the place in the top half.
Why???
Because I'm short. Those larger cup bras are meant for women 6' tall I think. So I take in 3 different sizes.
You know how your bra feels around. For instance if it's a 36 C but the cup is too small. The next option would of course be a 36 D.
When I went to try on bra's I'd take in a 36D, 36DD, and maybe a 38C, 38D too just to see if putting the hooks on the smaller hooks would make it fit well.
There are many ways to find the perfect size for yourself and not involve anyone else. But YOU have to go try stuff on to see how it feels.
One thing I've had to come to terms with is the style of bra I wear too.
I have always worn low cut demi cups with underwires in DD. I can wear any neckline with that style bra and not have it show. Plus the girls look sexy in that style even if they jiggle a bit when I move.
Now that I'm heavier and older I had to finally give them up for every day wear and I'm so much more comfortable. I wear a bra from Lane Byrant and I can walk in, try on each bra I'm going to buy and maybe one of them will have some odd fit to it but overall I try on every bra that I buy, just in case.
I get a bra that has good support and is comfortable. I don't dress in old lady bras either. I buy cute bras but without all the padding and wires.