The most straightforward approach is to take an OD cut and a face cut on a piece of stock. Enter the Z coordinate for the face cut into the Z tool offset table. Enter the X coordinate for the face cut into the X tool offset table and then add half the measured diameter of the OD onto this. That puts X on the spindle centerline.
NOTE: In saying add half the diameter I am assuming your machine is going to display machines coordinates as radial values for the X axis; if the machine displays the X position in terms of diameter then it is the full measured diameter that is used.