I have only been using the Johnson controller for around four days, but have found so far:
A. Beer temp preferred (by me) at 54F
B. Controller set to cut-out at 52F
C. Differential set to 3F
D. Controller probe hanging in the air in middle of keeper
E. "Beer" reading by temp probe attached the side of keg with plumber's putty
In my 64F'ish basement, my freezer kicks on maybe once a day, and I am seeing a keg temperature fluctuation of less than 1F.
I am sure this will change as the keg empties, and as the weather warms up.