You have two options:
Set the set point to 40 with the jumper in the "cut out" position. No matter what differential you use, the compressor will stop running once it cools down to 40. Then the temp will rise and the compressor will kick in once it warms up to 40+(differential) to start cooling again and bring it back down to 40.
The other option would be to set the setpoint at 40+(differential) and have the jumper set to cut in. In this case, for example lets say your differential is 5, the set point would be 45, so the compressor would start running when the temp rises to 45, and it would stop running when it cools by 5 degrees to 40.
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