WOW… This reminds me of an old fashioned snip test. We would shoot the entire roll at the same exposure, but before processing it completely, we would get a tiny portion (8″ or so) cut off the end to evaluate the exposure and the processing. If it was little dark, we could push it up to a stop in processing. If it was a little bright we could pull it about a half stop to lower the exposure. While this has no chemistry, it allows the same result – perfectly matched exposures across the entire take.
I love atmospheric shots and have played with several ways to make fog. None of them to any sort of satisfaction. This one works great, and when you apply just a bit of film grain (teensy) it becomes so much more filmic and realistic. This is quite a good tutorial for adding fog and atmosphere to images that may not contain enough for your taste. I think it can be over done, but with a light hand and careful layer opacities, it could work out very well.