The problem is that there is no air circulation in the freezers. The cooling coils are usually built into the walls of the freezer. What happens is that the cooling coils will get very cold and so will the walls, but the air temperature takes a long time to cool down. When the air finally cools down to your set point (42F) and the compressor shuts off, the air temp will continue to drop as the freezer walls are still very, very cold. To compound the problem, the air will be coldest near the bottom of the freezer, but your probes are near the top where the air temp can be as much as 10-15 degrees warmer at times. I've found the best solution is to install a muffin fan in the freezer to circulate the air which will even out the temperature a lot. This mod also has the added benefit of keeping your beer lines and taps colder. I've got my serving kegerator freezer set at 40F with a 5 degree differential. I have fans installed on both my kegerator and my fermentation fridges/freezers. Most refrigerators already have a fan in them, but not all do. The fans are a cheap and easy mod and solved the problem for me. My kegerator compressor is off 63% of the time and on for 27% at those settings. IIRC, the ambient temp was about 74F when I timed the cycle. Collar design/insulation and the room temperature will affect the run times. I measure the actual temperature of a poured beer and make adjustments accordingly. With the fermentation chambers, I use separate indoor/outdoor type digital thermometers to monitor the actual temperatures. I cover the thermometer probe with some bubble foil type insulation and tape it to the side of the fermenters. The theory is that this will provide a reasonably close approximation of the fermenting wort temperature without having to resort to a thermowell arrangement to measure the liquid directly. I've had excellent results doing it this way for a long time.