Digital thermometers are only ~13$ at walmart. Did you get a stick on thermometer? They are decent for reading ambient temps but what you really want to be measuring is the temps inside the hides as BPs will spend most of their time in them. Digital thermometers have probes that can be placed inside the hides so they work wonders!
The warm side hide tends to be hotter than the outside hot side temp because it traps in the heat. If you can't get a digital thermometer you should at least place your thermometer inside the hide. Just trying to give you some advice from personal experience here. : )