Chris Colohan wrote:
2. Couldn't you calibrate
your gages by filling the tanks, going for a flight, then sticking a
ruler into the tank to see how much is left? Is running the tank dry
any more accurate or useful?

If only it were that simple!
Fuel tanks are of various shapes and sizes.
Linear measurements apply only to tanks with flat sides, tops and bottoms.
I can tell you that on some airplanes, if you look in the filler neck
and see it within an inch of the top, you can still get 10 or more
gallons into it.