The standard formula is 1/12 of a pound of fuel per hour per unit of horsepower at cruising speed. Your light boat and four stroke will help. Gas is 6 pounds a gallon. You’ll burn 8.33 gal per hour. So if your cruising speed is 25 you’re getting 3 miles to the gallon. Any error would be in your favor so I’d expect 3-5 miles per gallon and maybe higher if your cruising speed is above 25.