If my measuring stick was off by an entire gallon, and I actually only had 6.7 gallons in the kettle, then I'd have 83% efficiency. But, I know I have really close to gallons in my carboy. And I measured what was left in the kettle by pouring it out into a smaller pot, and then into the measuring jug. There was 1 gallon left. That's consistent with what I measured.