I've arrived at the point where I want to establish personal film speeds for my 2 or 3 most used films. I don't have a densitometer, and after some asking around town, haven't been able to locate one to use.

So, I had this thought. If the goal is to establish a Zone 1 density, could one use a light meter to do this? Would this work:

1. Meter a gray card in even, diffused light. This should be Zone V;
2. Make a range of exposures from Zone I to Zone IV;
3. Make an exposure or two with the lens cap on. This should establish base + fog. Process the film.
4. Meter through the base + fog frame, then through the Zone I frame. If Zone I meters one stop different from the base + fog frame, is this now a true Zone I?
5. If the difference is more than 1 stop, it would seem that the film was rated too slow. If less than 1 stop, than too fast.

What's the downside of this? What am I missing?