There are two types of Datum errors, checking for motion issues when jogging the machine and checking for motion issues after running a job. The latter is what Chad was referring to and what you indicated by the .2 discrepancy after completing a job. To check the first type, after pressing Datum, jog the Y axis back and forth in a rapid motion at varying lengths and then pressing Datum again to check for error messages. If no error messages then that indicates the table is square, and cassettes are traversing up and down the linear guides with no issues. If you are only getting Datum errors when running a job, then it is most likely an issue with your cut file or the configuration file you are using, with the cut file you can try and go to Arrange > Convert to Polyarc before you create your tool path.