DonM a few years ago I picked up an electric gauge box, that's the best way to describe it. It's called an edge insight, it plugs into the OBDII port and gives me access to information that the computer reads that is not displayed on the instrument cluster. The things I want to keep an eye on are exhaust temperature, boost pressure, fuel pressure, and fuel level. The edge insight displays the fuel level as a percentage. I do my best to keep it over 10%, that said I have had to push it and the lowest I have ever run that fuel level was down to 1%, the fuel gauge was on empty and the needle below the E for almost 30 miles before I was able to get to the boss and get more fuel.
I end up using the fuel percentage when I need to fill up. I know my fuel tank capacity, and can figure out how many gallons I need.