I wouldn't be so confident in your predictions. We don't know w/absolutely certainty how full the tank is filled each time, unless you're visibly filling it to the brim each and every time. The Gas Gauge Says Full, But That's Not Quite True: NPR | PriusChat might be insightful, even though it's a Ford engineer speaking.
No, you are right. I don't know how much gas goes in. But I do trust the float to be consistent with the amount at the light. That is independent of the amount of gas put in the tank.
It's less horrible than the non-hybrid equivalent. The car gives you a gauge, similarly accurate to other vehicles, then tells you to fill up both with an audible sound and continual flashing visible indicator. The only thing the car doesn't do, is forcibly drive you to a nearby gas station. The good news is that if you do completely ignore all the warnings and hazards and actually run out of fuel, you can run on battery power alone for a short distance to pull off the road safely instead of just stalling in the middle of the road.
But if they're the type to have ignored all the previous warnings, what makes anyone think they'll be aware of the fact that they're depleting the last few feet of energy they have and won't wind up stopping in the middle of the road anyway?