An excellent explanation of the FBA - By using your upper bust measurement, you will be choosing a size that fits your frame, not your fluff. The top should fit nicely around your neckline and shoulders, avoiding the droopy shoulder seam, which is common when you make a size too large. Next, you need to know your full bust measurement. The difference between your upper bust and your full bust measurement is the amount that you need to add to your pattern.
How to measure an armhole princess seam