I have a calibrated eyeball. When in a Navy school, the instructor wanted to prove a point about how bad people are at estimating. He called six of us to the board at the front of the class and said, "Draw a line eight inches long." Then of course he took a ruler and masured each attempt. I still remember how ridiculous the spread was: 5.25, 9.75, 9.5, 6.75, 7.25 and when he measured mine, he didn't say out loud that it was 8.0625. He just turned around, looked at me and gave me the finger.