If you have a remote
infrared thermometer, you can take the temperature of the oven at various times after lighting
it up, and correlate the readings to calibrate the existing gauge.
If the sensor for the built in thermometer is all dirty, cleaning
it may help. If the present gauge is regular, like it always says 300 deg. F. and is actually 400 deg. F., you'll be able to work
it out. If it varies, though, you might try and get another (no guarantees) thermometer.
I feel your pain with this one. My oven's thermostat doesn't work
, so that I am checking its flame and temp. every 10 min. throughout a baking time. And there are things I daren't try to cook, because it is so finicky.
Good luck in getting a workable solution.