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Larry --
The "Personal Reference Speedpoint" is intended to do two things: first (as the name implies), it establishes a standard reference point in the film family charts that defines the "ISO" speed point, as nearly as we can calculate it. We don't have access to the lab equipment and procedures that the ANSI uses so we have to start out with the assumption that, at least in some instances, the official ISO numbers are "correct" and reliable for field use. Unfortunately, the real speed of most film/developer combinations is not indicated by their assigned ISO numbers, as we can prove empirically by careful field tests (as an obvious example, most photographers will probably agree that the old TXT, developed in D-76, was not an ISO 320 film).
The official ISO numbers are derived from the exposure required to produce a negative density of 0.1 over B+F when a subject range of 1.3 results in a density range of 0.8; in other words, when the average gradient is approximately 0.62. We can't duplicate the test conditions that ANSI uses to arrive at that standard calibration curve but we can recognize it when we manage to create it by whatever means. That's what the Plotter does when it constructs the "ISO triangle."
The second thing the PRSP does is position the curve family more or less optimally in the graph space. Since that standard gradient is established by development, rather than exposure, the Plotter can find the standard curve in any family that contains curves whose gradients straddle that critical 0.62 value—regardless of the film test exposure or the speed of the film. The trouble is that changing the test exposure will shift the entire curve family laterally in the graph space. The Plotter won't care; it'll happily identify an "ISO speed point" wherever the curve family happens to be because all it's looking for in that magic 0.62 gradient.
This is no problem at all except for one practical matter: if the test exposure is too great the curve family will be displaced to the left so far that some or all of the curve toes will be chopped off; and if the test exposure is not great enough the family will be shifted to the right, shortening the useful curve length and restricting the SBRs. In other words, it's desirable to adjust the test exposure to place the curve family far enough to the right to keep the toe contours intact, but not so far that we lose useful curve length. This compromise position is usually established when the Plotter's "ISO" speed point falls on, or near, 2.4 on the graph's x-axis.
The actual number doesn't matter; the important thing is to pick one and standardize on it as your Personal Reference Speedpoint. I recommend 2.4, and that's the default value the Plotter uses, but you can change it if you want to.
The confusing part of this is that the PRSP represents the correct "ISO" reference point for ALL films, irrespective of their individual speeds.. This is possible because, once you've established a test exposure with an "honest" 400-speed film (for example), increasing that test exposure by two stops will be correct for a 100-speed film. In other words, the curve family location depends on two things: test exposure and the actual speed of the film. The families of all films of the same assigned ISO value, if given the same test exposure, should appear in the same location in the graph space and share a common ISO speed point (as found by the Plotter). If the speed points are not the same, it has to be because the film speeds are not the same.
Obviously you will want to establish your PRSP as accurately as possible and this is where some educated guesswork is involved. By laborious trial and error (and careful analysis) you can determine whether the assigned ISO values for various film/developer combinations are satisfactory, and when you've selected materials that you consider to he honestly rated, run them through the Plotter to find out where it places the ISO speed point. If it's not on, or very near, 2.4 (or your chosen value) adjust the test exposure to correct it. When that adjustment has been made, preserve those test conditions as "standard" for that ISO speed.
From then on, adjust the test exposure as necessary to be correct for the nominal ISO speed of each test film. The Plotter will then tell you what the real, effective film speed is.
I don't think 100 TMax in D-76 is a good combination for establishing your PRSP. I'd recommend it in DD-X; or 400 TMax in DD-X or TMRS or Xtol. Use a convenient dilution for your temperature. Within reason, dilution doesn't affect speed very much.
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