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Claude --
It's probably not helpful to involve N-numbers in any BTZS discussion because they have no consistent relationship to the more reliable values of ES, DR, SBR, and G. In fact we probably shouldn't have provided any N/Dev or N/EFS data in the Plotter program, because in order to make them work at all in this context, we had to specify a "spread" value that's not part of the traditional Zone System calculations. Of course the problem is that ZS users assume (logically enough) that these N-numbers are the ones they're familiar with. In fact, they MAY be—but they probably aren't, and that's why we don't like to deal with them.
The ES, DR, SBR, and G values we use are all objective and measurable. ES and DR are really the same in these calculations; ES defines the range of exposing light intensities the printing paper needs in order to produce a full range of image tones (in other words, it corresponds to the paper's contrast grade). DR defines the range of negative densities that, when transilluminated by the printing light, will provide that range of exposing light intensities the paper needs. All of which simply means that when ES and DR are the same, the negative is a good match for the paper.
These relationships are based on a right triangle. The SBR is the triangle's base dimension; ES (or DR) is the altitude; and G relates to slope of the hypotenuse. Knowing any two of these values you can calculate the third with these equations:
G = ES/SBR, ES = G*SBR, SBR = ES/G
Of course all values have to be in the same terms—usually logs or stops. I'd strongly advise that you leave "N" out of these calculations.
The rate of G increase with development time depends on several factors including film type, developer type, dilution, and temperature. It can range from very rapid (a few seconds makes a significant difference) to negligible (no significant change for several minutes). In other words, this is another variable that is not universal, but depends on specific conditions.
Hope some of this helps. I think these relationships are adequately explained in the book, and no doubt other forum lurkers can offer useful insights based on their own experience.
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