The only thing that seems to be missing from the equation is a constant.
That constant can only be determined one way, by experimenting to see what effect a/any given volume of light has on that film.
The actual reference point though to key a meter to seems to be pretty arbitrary. Any given density on the negative could be used as long as it is repeatable.
The only thing missing is the constant we "found" experientially for a given film's speed.