While in theory for some tests 1.001Mohm might be a pass and .999Mohm might be a fail, the reality is you are generally looking for hundreds of Mohms and anything vaguely close to the expected threshold needs investigation.
Insulation also varies significantly with temperature and moisture/humidity. If you aren't adjusting readings for temperature and humidity, there is little point caring about calibration certificates etc. It is worth having some high value resistors in the bag with the megger so you can verify the device is operating correctly.
