FWIW, I had a heck of a time calibrating my eheim jager heaters in the past as well. It appears you got them calibrated correctly. When you moved the heater to your tank and you saw it turn on, this is not abnormal. I find my eheim heaters will cut on when moved, but they only do so for a short while. Yours may have have been staying on, though, so I don't know. Regardless, you have a new heater now, so it is no longer an issue for you to worry about. However, I would still highly advise you to getting a heater controller (Ask me how I know or ask my well done harlequin tusk I baked in a QT setup). Set your heater a degree or two above your target temp and let your controller do the turning on and off. If the controller were to stick on (which is less likely), the aquarium heater thermostat also has a chance of cutting off when it reaches the higher temperature. This way you have some redundancy. Ideally two smaller heaters attached to a controller would be even better.
Also, your tank temperature may climb during the day above your set temperature depending on what pumps and lights you are running, as those things will add some ambient heat to the water. My tank temperature rises about 2 degrees due to my T5 lights, return pump, UV, and skimmer pumps adding extra heat to the tank. In your case, your heater is probably only needed at night much like mine. I'd be curious what temperature your tank would drop to even without the heater (not that I'd recommend not having one).
