For $7 more dollars, I would get this thermostat instead of a dial rheostat. With the dial ones, you always have to adjust the dial in order to keep proper temps with a fluctuating ambient temp. With this thermostat ( http://www.amazon.com/Hydrofarm-MTPR...arm+thermostat ), you just have to set the temp (I usually set mine to 89) and it will keep a much better temperature.
Take the bottom reading because that's where you're ball python is living. 73 is a little too high for everyday humidity. You just need it to be around 50% unless your ball python is in shed. The humidity will gradually fall so it shouldn't be a problem. Just realize that scale rot can be caused by too much dampness of your substrate. So if you're constantly misting and your substrate remains damp, then you can still get scale rot even if your humidity is on par.