I watch a lot of HVAC repair videos on Youtube and the technicians frequently reflect on thermostat settings, indicating that these systems aren't meant to be running at temperatures lower than 68 deg F. I wonder if you may have uneven airflow in the various parts of the house causing you to have to lower the overall thermostat to compensate? We have one bedroom on a longer run that doesn't get as cold as other rooms, but we were able to tweak the registers to provide more airflow to that bedroom.