expensive laboratory graduated cylinders are +/- 5% accuracy. I'd expect your 1 or 5 gallon jug to be much worse. using a small vessel to measure volume multiple times increases your error even more.You're thinking of the error on flasks and beakers, which is due to mass production and not being designed for measuring. It is easy to get better accuracy in large vessels if you calibrate each one yourself. 5% of 5 gallons is 1 quart - that's 1/2 inch of water in my brewpot.