Appendix B. 100 Collected Rules of Thumb to Help Estimate Signal-Integrity Effects
A rule of thumb should be used only when an OK answer right now is more important than a good answer later.
A rule of thumb is only meant to be a very rough approximation. It is designed to help feed our intuition and to help find a quick answer, with very little effort. It should always be the starting place for any estimate. It can help us distinguish between a 5 or a 50. It can help us see the big picture, early in the design phase. In the balance between quick and accurate, a rule of thumb is quick. It is not meant to be accurate.
Of course, you can't use a rule of thumb blindly. It must be coupled with an understanding of the principles and good engineering judgement. ...