Clamp the workpiece in the spindle.
Load an outside diameter turning tool into the tool turret.
Close the lathe door.
Make a small cut on the diameter of the material clamped in the spindle. Approach the part carefully and feed slowly during the cut.
Jog the Z Axis away from the part. Move far enough away from the part so that you can take a measurement with your measuring tool.
Press Spindle STOP and open the door.
Measure the cut that you made on the workpiece.
Press SET X  to record the X-Axis position in the offset table.