The drop in range will depend on how efficient your heater is & how cold it is outside.
Some of the EVs have resistance heaters and they can use quite a lot of juice.
Some of the EVs have a heat pump and they use relatively little juice.
BMW i3 – resistance heater (heat pump is an optional extra)
PSA EVs – heat pump
Kia/Hyundai – heat pump in the UK market
The largest drop in range is from the lithium batteries being cold.