• The math is absolutely correct, Dann.
Have you checked whether you get different results using Emmanuel's math?
(And if so, Emmanuel may try to point out the errors. Which will be in the application of what Emmanuel says you should do, for i'm sure there are none in the maths i use.)

What matters in this all is how much the 'light source' is moved away from the film, relative to where it was (at infinity - to provide a good starting point). That light source is the exit pupil of the lens.

The pupil magnification is a measure for how much the exit pupil to film distance differs from the focal length. So it can be - as i have described - be used to calculate the exit pupil position. (Made easier if the lens manufacturer provides the diameters of both pupils).

From then on, it's plain sailing, easy application of the usual, well-known formulae, using the exit pupil to film distance instead of the focal length (the use of the focal length is based upon the thin lens assumption, and that only to simplify things, at the cost of accuracy).

And these formulae are simply an application of the inverse square law.