you could just use your trip odometer and gauge how many miles before you need a fillup
20 gallon tank times ~15 mpg on the conservative side ~ 300 miles
so just start thinking about fueling up at about 260 miles
ps since you ran out of fuel so many times, you're shortening the fuel pump's life
fuel acts also as a coolant for your in-tank pump
pps you're probably going to prematurely clog up that fuel filter as well since sediment does find it's way to the bottom of the tank over time, so change that out as well
