The specifics are fuzzy but the range shown at any time is calculated from a combination of your current trip efficiency and recent usage. If you made it home on Friday with 1 mile left on your range you definitely would not have made it without charging. The less charge in the battery, the less off the estimate can be in absolute terms. Though when the car hits 0% you do have an extra 1kWh of emergency reserve. Something to keep on mind - I actually had to use that yesterday myself.
Try to think of it in absolute numbers whenever you can. At any time, the car is estimating that you will have some particular average efficiency and it's simply telling you how far you can get with that efficiency and the remaining charge. Switching between surface streets and highway, uphill/downhill and even with and without climate power will change the values. Since the climate stuff averages over time the car will also estimate that usage and just subtract it from the amount it is using to calculate available range.
I make it a point to use the "MyView" setting on my left dash screen with Budget + Average with gauge scale on the right and accessory power on the left, no budget text.
That range gauge has some very useful characteristics. At any time, the line across it shows your instantaneous Wh/mi and the ticks on the side show your average since the car was started for the current trip. If you aren't using nav to a destination, the highlighting on the gauge shows you the efficiency the car is using to estimate your remaining miles on your charge. If you are using nav it shows you the efficiency you need to average to make it to your destination with your remaining charge.
So if you consistently hold that line over or under the ticks, the ticks will move closer to it. If the ticks are inside that "range cup", not only are you meeting/beating the estimate, you can estimate how much better you're doing. Say your car shows you have 40 miles left and the range cup shows 300Wh/mi as the estimate, but you have the ticks sitting around 270Wh/mi and don't expect that to change. That means you're 10% more efficient than the car is guessing and you can expect to get about 10% more miles than it says. About 4 miles.
Depending on how it updates during your drive, that could mean that at 4 miles in it could say you have 36 left (no significant update to estimated efficiency), 40 miles left (efficiency estimate updated to "match" your actual recent efficiency) or somewhere between.
You can see on mine it's estimating 98 miles with an average of just under 300Wh/mile. Since it's been sitting, the efficiency for the trip so far is closer to 450, but that's not what I'm actually getting. But if I was, that means the estimated efficiency is 2/3 of my actual efficiency and I would expect to get ~66 miles on my remaining charge.
I really need to do a full illustrated post about this some day. I wonder how many people even know about that view, let alone the actual mechanics. If you have a feel for your typical efficiency on different routes, you can get a good feel for how much range you get from a given charge and start ignoring the meter.