here's how it works:
The army value is actually a simple multiplication of the parameters of a unit except the population required.
The efficiency is calculated by this
log (food per hour) to the base of (army value)

As for Tybalt, I saw the thread. Very impressive AND useful. However I am using a generalized formula, while you r paying attention to specialized attribs of the units, i pay attention to all of them.
Eg. A ballista is powerful and has long range but is slow.
A scout is useless as a combat unit but awesome for rapid-fire atk on low-walled villages and towns.
A batram has awesome life but eats 100(F!) food per hour.
The same logic applies for all units.

The case with archers:
They will die a horrible death if there is no meatshield, however WITH a meatshield, they r very close to the best.
Also Tybalt, you hv taken the sum of all resources.
Problem with tht is ...on my server iron costs 15 gold but stone costs .4 gold.
So a simple summation of resources like tht is not really effective. However, replace the "resources" with food per hour. It'll be way more useful(just a suggestion).
If you still think i'm an idiot, post it here and let me see wt i can do abt it!