Michael;

The paper sees the base + fog, and so lets exaggerate this situation to 2 films with a Dmin of 0.2 and 1.0 respectively. Since they are vastly different, you would not print the foggy one at all. Subtracting base + fog might show that they have exactly the same curve shape though, so what might you think? I would probably discard the foggy film shot regardless of the match without for because the foggy example would take 3 stops more exposure to print through the film.

So, in this example, with a foggy film in one case, the example without B+F would appear to match the two curves, but in reality the foggy example is not printable.

Besides, if you slide curves around, as I noted earlier, then you can do the sliding for speed, curve shape, and B+F or minus B+F just by sliding the two sheets of paper.

I use a light table for this type of work.

PE