I'm becoming quite fond of this rule. I carry it about with me in my briefcase, or in my satchel when I'm on the road. I use it mainly for Miles/Kms conversions as well as calculating fuel consumption and ETAs.
When I'm in a car with no speedometer (which is often) I can get rough estimates of true speed fairly quickly. We don't want any more speeding tickets!
I confess to scratching "gal,km / m,l" and "FC" in appropriate places to assist in fuel consumption conversions. Please forgive me. :-)