How accurate is the thermometer? Many inexpensive digitals have specs at +/- 2ºC (loosely 4ºF) some are better and have 2ºF in error it is possible for the swing to be out by 6 either way or 4 either way. Incubators need accuracy you could buy a thermometer that is around the 0.8ºF +/- accuracy or a thermostat that is that accurate (herpstat, helix, ecozone, herpkeeper all do) at the end of the day either way you will spend around 120$ or either stat or thermometer.