We normally use around 2 ounces per gallon, it's a judgement call really because the tank never gets completely empty, there is always 2-4 gallons left in our 15 gallon tank whenever the sprayer stops spraying, so we eyeball it most of the time, but we try to get around that 2 ounces which we know is more than enough, I think recommend is only 1 ounce per gallon.