When do you turn off the heater?

If your room temperature rises above the setpoint temperature, the water no longer transfers energy to the room, the return temperature rises, the heating regulates down or switches off. So you don't need to adjust anything anywhere.

We have no ERR in operation and the heating threshold is set to 13° outside temperature. However, I cannot yet say if the 13° is appropriate or too low/high.

Our heating usually turns off quite reliably at 9-11° outside temperature. Otherwise, it generally runs for days or weeks and modulates by itself.

Ok, understood. Thank you