To find your sewing cup size you simply subtract your high bust measurement from your full bust measurement.
So a high bust of 36 and a full bust of 39 = 3 inch difference
The general rules for sewing cup sizes are as follows.
These are basic rules Not the law!
1 inch (2.5cm) difference = A cup
2 inch (5cm) difference = B cup
3 inch (7.5cm) difference = C cup
4 inch (10cm) difference = D cup
5 inch (12.5cm) = DD cup
6 inch (15cm) = E/F cup
7 inch (17.5cm) = G/H cup