It is impossible to calculate the killing effciency of a unit.
The variables are so many...honor, terrain, formation commands, flank/rear attacks.

The elasticity.

As a matter of fact, a practical way of calculating the killing efficiency of a unit is to check out the results of a battle with the battlefield calculator by Istvan.

That would only be THEORICAL, being the situations too many to face for a standoff ranking of the unit killing ratio.