>Another old carpenters trick uses a ruler/yardstick/tape measure. Place the zero end of the device on one edge, and raise the opposite end on an angle until you reach a whole number that is divisible by the unit you want.
>
>For instance, to divide 100 inches into 3 parts, place the zero on one side and move the rule up until it reaches 102. Then make marks at 34 and 68.

Very good technique and mathematically correct. The error may come from dropping the marks over the short distance, but that can be cured by taking a good right angle.