Ok I was coincidentally thinking about this just today. The values of lux-s, or actually log(H), are measured at the film plane like you said. So whatever the actually luminance of your scene is, it's going to be a lot less once you are talking about actually light reaching your film. Where the density rises 0.1 above the film base + fog (fb+f) in terms of log(H) is determined by the ISO speed, by H=.8/ISO. Take the log of that, and that's where your film density = fb+f+0.1.
I was curious as to where the nominal metered value (of 18% gray) is. Turns out it's at fb+f+0.1 + 1.0. Or 3 1/3 stops up. So you basically get 3 stops below and 6-7 above, depending on the film.
I wrote some of this up quickly and attached a pdf of it if you're interested. Of course, it could all be wrong, but I think I got it right
