It's non-arbitrary, but neither is it precise. 100% is clearly too high, and 10% is clearly too low.

And since I started calling it The 40% Rule fifteen years ago or thereabout, a number of my friends and acquaintances have embraced the rule in this incarnation. Obviously, some things are unquantifiable and the specific number has rather limited application. But people like it at this number. That counts for something - and it gets the message across in a way that other formulations don't.

Some are nonplussed by the rule, but the vigor of support by some supporters gives me some thought that I picked a number people like. Since I never tried another number, I could be wrong - but I don't think I am.

--JRM