Eheim Jager heaters are one of the only heaters available for aquariums that have a built-in calibration function, and most heaters are relatively accurate, but we have found that some can be off as far as 4 degrees. Even though the Eheim heaters were one of the best as far as temperature deviation, having the ability to calibrate them makes them even better.
Calibrating the heaters can be done either in your tank while they are running, but we highly suggest calibrating them in a 5-gallon bucket since it is a much smaller volume of water that can be heated or cooled much quicker, the only other thing you will need is a reliable thermometer or temperature probe. Once the heater has heated up the water to the indicated setting, you can use the thermometer or probe to read the temperature of the water, and if it does not match the heaters setting, then you will want to turn the red indicating arrow until it matches the same temperature of the water. That will allow you to accurately adjust the temperature of your aquarium without having to compensate for any deviation that the heater may have.