I live North of North Dakota which has similar cold experiences. It will be reaching -40 here this weekend, around -50C with wind-chill. Wind-chill does not affect engines except to make them lose heat faster. But -40 is -40 and wind won't matter to metal.
As noted, 60% is what we do up here. A simple coolant test will show that. Last thing you want is it to freeze and expand and crack the block or something. If you're lucky it will turn to slush and not freeze solid, but it might freeze solid and at minimum bust the rad.
Plugging in the block heater will help the engine but not the rest of the areas that the coolant is in.