Any question answered...

Is there a method for deriving a persons optimal crank length from simple measurements, easily obtained with a tape measure?

I found one on-line which simply took your inside leg measurement (34") and spat out a result of 172.95mm, but that seemed a tad facile.

What do you want it based on, if not inseam?

http://www.machinehead-software.co.uk/bike/cranks/cyclist_crank_length_calculator.html

Buy a 172.5 and forgetaboutit.