Gene,
I couldn't agree more. I put 280 hours on my R 25, with Cummins 150, before selling her. In all that time, none of the fuel gauges worked. Didn't bother me, occasionally I laughed at how wrong they were.
But I always kept a Log Book, down to 15 minute intervals, and every day or two, I would make a running total of Hours on that particular previously full tank.
I found that, over a full tank, I always averaged less than 2 gallons per hour of, with varying uses and speeds.
Then, as you stated, you always need a significant reserve. The tank held 75 gallons; I allowed a 25 gallon reserve.
So, 50 gallons at 2 gallons per hour, it simply meant that----whenever my total hours on the tank approached 25 hours, well I headed for the nearest fuel dock.
Digital gadgets can drive you crazy. A stick down into the fuel tank isn't possible.
Keeping a highly accurate Log will save the day, every time.
Charles