If you have a plastic tank and can see the fuel in the tank.
You can shine a flashlight in to the fluid portion and the level of fuel in the tank will be revealed as the light bounces of the surface of the fluid. No measure the fluid level in inches. Measure the height of the tank.
Device the tank height into the fluid level anf you get the percentage of fuel in the tank. Check the tank volume (usually in gallons of liters on a tank label. Sometimes embossed in the plastic.) Multiply the total volume by the percentage and you have the number of gallons of fuel.
Once you know the fuel level you can begin recording the hours and compare against the fuel volume consumed. Generally .5 to .7 gallons per hour of engine run time. Using the hours of engine run time you can accurately estimate your fuel level. It will be more precise than any gauge you can install.