Enter destination in the navigation system--it will estimate battery remaining at destination and also remaining after a round trip back to start. These estimates will change as you drive. So you can see if you might run into trouble. Also, if you are going to need a supercharger stop, nav system will include this in your routing, even if the need is due to more energy expended than expected, not realized until the middle of your trip.
WRT buffer, the winter is tricky as you'll see. But if the weather is 50 or better, 10% is plenty.