Put a tooling ball  attached to a magnetic base along the X Axis, near the edge of the table.
Move the position of the base so the ball arm is at a 45 degree angle to the t-slots.
Make sure that the tooling ball  is tightly attached to the magnetic base and that the magnetic base is flat on the platter surface.
Jog the Z Axis until the probe tip  is 0.1" (2.54 mm) above and centered over the tooling ball .
Enter the tooling ball  diameter.
Choose Output to MDI and start the program in MDI.