After reading my posting on the Rule of 60, Malcolm Teas kindly pointed me to a 1996 Usenet posting by Barry Silverman (originally written ten years ago, in October 1994) describing the French method of teaching pilot navigation. He also recommended a couple of books on mental math for pilots, for those of us who wear propeller beanies over our headsets — if you’ve read this far, you know you’re one of us.
The French method divides 60 by your true airspeed (there’s no escaping the number 60) to get a factor F, which you can use in mental calculations to estimate time enroute and wind correction angle without requiring an E6B or calculator: see the link to the original posting for details.
Malcolm also recommended two books for people who enjoy flying by numbers: Diversion Planning: How to Navigate Around the World with a Stopwatch and a Pencil (USD 8.95), written by a retired British RAF navigator named Martyn Smith, and Mental Math for Pilots (USD 27.95), written by Ronald D. McElroy, Pam Ryan, and Carol Core. To these, I’ll also add the Cross-Country Flying chapter of John S. Decker’s excellent (and free) online flying text, See How it Flies.