Sensitometry and lens transmission formulae

I'm planning on doing a bit of sensitometry and am having trouble finding a formula that will tell me how much light a lens transmits, in compatible units at each side. The reason I want to know is that I want to calibrate my (flash) sensitometer using a flashmeter located where the film-under-test will be.

Consider a simple imaging chain: light source, subject, lens and sensor. I take an incident meter reading at the subject (Es) and an incident meter reading at the film plane (Ef). What is the ratio of Ef/Es as a function of the lens' aperture?

The only reference I could find is this one for CCTV sensitivity analysis, and it says:

Ef/Es = 0.2 * t * R / (N^2)

where t=lens transmission (any non-ideal filter-factor built-in to the lens due to imperfect coatings etc), R = scene reflectance and N = f-number. The presence of that 0.2 is at best highly unsatisfying. Given that their example has R=0.89, would I be right to suspect that the 0.2 is a poor approximation of the 18% reflectance standard therefore R=1 or R=0.9 for an 18% card? Or should I assume that if the subject is an 18% card and the lens is ideal, Ef/Es = 0.036/(N^2) ?

Any better references welcome...