If you are out of a size:

Go up a size, down a cup or go up a cup, down a size

ex.  Customer's size is a 36 D 

Two options:  you can give her a 38 C or 34 DD

Extra Fitting Advantages:

Your customer has a long torso and she is a 34 C CUP

34 C-36D

To determine the proper cup size- find the difference between step 1 and step 2

Step 1: Over bust measurement is 34

Step 2: Full bust measurement is 37

Step 3: 37-34= 3" difference

Cup size is C