Picking the right size for a petite curvy girl!!!!

HI ALL:: Long time no chat! I love the Marais. Is it better to pick the size based on upper bust or full bust and do FBA/grading? UB 37 (sized M) FB 42 (between L and XL) Band: 37 (sized M) Waist:39 (XXL) The fit point on this seems to be shoulders and bust. Do I start with a M in the shoulders...Then do a bit of an FBA on a D cup? Then grade out for the voluptummy?