I think you will find that the controller shuts off as soon as it hits 18 degrees. I have the same controller on our beer trailer and it does the same thing. Basically it cools to a FULL 1 degree below the set temp and that often causes an overshoot to 2 degrees for a few minutes (in my case it is an air conditioner).
The Fahrenheit should narrow your band down a lot and is really your only option with this type of controller.
The River Company Restaurant & Brewery, Inc