So Neiman Marcus has this little gizmo that asks you your size in certain designers, then predicts what size you'll need in a item.
http://www.neimanmarcus.com/p/.....at=product
Look at the Your Best Fit button on the lace top.
I don't think it works very well, I'm a medium in Vince and a large in Robert Rodriguez and it said I'd need a medium in the DVF top. I know I'm a large or 12 in DVF because she runs pretty small.