Got a starter kit with heat mat underneath the tank and three probes inside (stat and 2 for the digital thermometer). I've set the stat to 28 degrees but the temperature display has shown as high as 34+ before cutting out even though I've placed a temperature probe next to the stat probe,so that they should be getting the same reading.
You will have to monitor the temp range your on off stat will give you and set the stat via the temperature reading on your thermometer. If you get a 6 degree variation then set the stat to switch off when your thermometer is at about 29 degrees.
Exactly. Keep your thermometer probe next to the stat probe and ignore the dial figures on the stat. Until you get into digital thermostats the readings on them are not very accurate.