Put the magnetic base of a dial indicator on the spindle head.
Set the indicator to zero. Indicate the platter or collet face along the Y Axis. If the error is more than 0.0003" (0.008mm) adjust Parameter 212 in small increments in the positive (+) or negative (-) direction to adjust the flatness.
To make sure the travel limits of the A axis are correct HANDLE JOG in the positive and the negative direction. Make sure the movement stops at 120 degrees in each direction.