Anthony, maybe you should test it at around 350, which is a common roasting/baking setting. Give it longer than ten minutes, like at least half an hour. Instructions for portable roaster ovens typically advise to pre-heat for at least 20 minutes. Your thermometer should be elevated off the bottom surface of the insert.
I'm guessing that any attempt to calibrate would only give you grief. I have a mid-'40s GE roaster and from the looks of its thermostat, I don't think there's any option to calibrate it.