Proper Sizing for busty girl??

So looking at the enmore, the upper bust sizing puts me at a 10. Full bust? 18. But no way am I that wide in the shoulders. So the 10 would be the better fit.. except.. While tons of ease? How much is there for purposes of the more buxom of us? Also my cup size is more an F/G than a D. Do I need an FBA on the D cup shelf? Would that effect the exterior of the dress? Do I FBA that too?