My hunch is that heating is pretty insignificant compared to the huge power needed to move the car, like single digit percentage. But I like your question at the end and would be interested in someone running the numbers.
edit: Ok here we go - Model 3 users report using about 240 to 300 watt-hours per mile. So I think that means a 1KW heater for an hour is equivalent to about 3 or 4 miles of range.
Note that the heater in the model S/3 is capable of ~6/4 kW. In most cases you won't be running the heater on full for an hour, but at -10 you might be. When you start talking about 12-24 miles of range per hour, that's how you get up to the worst case of ~20% range loss.
Still, that just means you want to limit your -10 degree trips to like 2.5 hours instead of 3 hours, and only if you don't have access to superchargers.
edit: Ok here we go - Model 3 users report using about 240 to 300 watt-hours per mile. So I think that means a 1KW heater for an hour is equivalent to about 3 or 4 miles of range.