I use the trip meter on my depth finder. I have a 25hp merc and figured at WOT I get approx 7-8 mph. My tank is 9 gallons. When my trip meter gets to 50 miles I refill. Most of my running is way less then WOT so it usually takes about 5 gallon to fill it. Just figure out your fuel burn at max and then decide far you want to run the tank down before refilling. Learned to do this when I was a kid. Our 1972 AMC hornet fuel gauge was broken so Dad got gas based on the odometer.