The thermostat, when properly clamped to the evaporator plate starts the compressor
when the plate reaches a temperature of approximately 15 to 45 degrees F, depending on where you set it. Obviously, part of that range is above the freezing point of water
and, when set that high, the plate will drip water until the compressor starts. If you don't want it to drip water, you need to set the thermostat to cut in below about 32 degrees F.
of the frost, that you say is 100% is a function of the amount of refrigerant in the system (100% is at least a full charge). The thickness
of the frost is a function of the door seal, the frequency of opening the door, and the humidity.
Hope that helps.