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.