I find rules of thumb fascinating. Would love to hear what other rules of thumb people use in their day-to-day.

Then they measured my right Thumb, and desired no more; for by a mathematical Computation, that twice round the Thumb is once around the Wrist, and so on to the Neck and Waist, and by the help of my old Shirt, which I displayed on the Ground before them for a Pattern, they fitted me exactly. -- Gulliver's travels