I filled up today, as I pulled to the pump I saw it read 1.0 gallons. I added 7.0 until the click - same pump I always use. Scangauge said I had used 8.5 gallons, pump said 7.0, gas gauge said I'd used 7.7. It's all within a gallon... and all that matters is, when the reserve starts flashing, get gas. If you ever trust a gas gauge on any car to be exact, you're taking a risk. Angle, air bubbles, all kinds of things will make an analog gauge sway + or - 1/8 of a tank... the Smart's gauge is just digital on the readout, but in the tank it still relies on the same laws of physics.
Run out of gas in rush hour traffic or 5 miles from a gas station and see how proud you are that you found the exact measure of the tank... you'll probably feel pretty dumb, worse if you get rear-ended in the middle of a highway.
There is no proof. The tank is full, half or something less than a quarter. Fill up. Keep a fiver in the glove box - you'll always have gas money. And with an 8-10 gallon tank it takes less time than getting a mocha.