See attached guide, needs to be performed when gantry is at the end of its travel on both North and South side of the table.
A good way to determine on a 4x00 machine is not in square or that the flags need to be adjusted is by DATUMing the machine in between jobs. This will give you a good indicator that your gantry needs to be adjusted. Also, which "guide" are you referring? When was the last time you greased those zerks on the Y drive cassettes?
Yeah I get at least .2 error u and y even in middle of jobs. Greased zerks and cleaned rails yesterday.
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.