I always used trial and error and managed to give exactly the amount my sons needed, wasting food isn't allowed in my religion and formula production is quite resource heavy, so leaving an oz in the bottom wasn't acceptable to me, think about how many extra bottles worth of formula are being thrown away or washed down the sink each year. If my sons drained a bottle but were not hungry again before 2-3 hours I was satisfied that this was the perfect amount, it is wasn't enough they would have been hungry again almost immediately xx