You need to get that thermometer asap
The thing is with dimmers/rheostats is that they rise with the ambient temp. So if your room goes up say 6 degrees, so will the belly heat making it possibly uncomfortable. You can't really make any assumption of anything until you get a thermometer with a probe. Go to walmart and get an acurite thermometer w/ probe there like 12 bucks. You should ALWAYS be monitoring how hot it is because things break, or things just happen that your not expecting. Whenever using a rheostat ALWAYS monitor it with probed thermometer, they aren't reliable sources of heat control.