A 1968 paper about mirror-making is a handy guide to a particular rule of thumb:
The paper is: “A Rule of Thumb Arranged for Ten Fingers,” W.P. Barnes and Frank Cooke, Applied Optics, vol. 7, no. 3, 1968, pp. 454-454. The authors, at Itek Corporation, explain:
To obtain a quick but useful estimate of the engineering sophistication required in the fabrication and testing of large mirrors, the following procedure is recommended:
1. Determine the diameter D which is usually given.
2.…
10. For subsequent calculations, omit steps 3, 5, and 6 and the latter two-thirds of step 2. Steps 7 and 8 will no longer apply. An appropriate punishment for those who persist in step 3 is suggested by the fact that both thumbs are now unoccupied.
What remains of this procedure is in fact useful.