I stand it up. If you fill it up leaning left, when it comes up vertical the bottom of that tube is In the gas.
Wobble it back and forth before the cap goes on.
You say you leave room, but the tube under the cap is the gauge. You can feel the bottom of that tube with the end of the nozzle.
If they have paper towels, grab a few, then it's not so bad.

I'm old. I always tried to make sure I didn't have to push it to the station.
You might want to get an Official Legal gas container to haul around.
At highway speeds, when it is time to go to reserve, it just shuts off. At around town speeds, it kinda runs a bit weird, then bucks and coughs. You really don't Want either of those when you're already on reserve.