You have to play around with it. Depends a lot on how it reads temperature and how much flow you have through the unit. In my case, for example, the temp probe is inside the chiller, and flow through is on the low end of recommended. As a result, the chiller cools internally faster than does the main tank so I have set the ranco for a 3 degree spread (77 to 80). The chiller does not rapid cycle this way. When the tank is above 80, the chiller comes on until the internal chamber drops to 77, then it goes off. It has not cooled the entire tank though, so the chamber temp immediately begins to climb until chiller clocks on again. Been running it this way for a while and it keeps the tank pretty stable at 80. Obviously if your unit measures temp in the sump you'd not want such large setback.
Unfortunately my chiller is too large to be able to run it through my apex for redundant control so I just rely on a low temp alert.