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Larry --
This can get confusing, but I'll try to explain. On the graph x-axis the numbers are step tablet density values and they increase from right to left. The thin densities at the right transmit a lot of the exposing light; the heavy densities at the left transmit very little, so the test exposures, transmitted by the various steps, increase from left to right.
If you were to test a 100-speed film and a 400-speed film, giving them the same test exposure, the two curve families would appear 2-stops apart in the graph space—the 400-speed family at the left, the 100-speed family at the right. If you want the families to fall at the same place you'll either have to decrease the 400-speed exposure (to shift the family to the right) or increase the 100-speed exposure to shift its family to the left.
In other words, the family's lateral location is determined (1) by the film's sensitivity or speed, and (2) by the amount of test exposure. Since the exposure TIME is a constant, you have to vary the exposing light intensity to compensate for different film speeds if you want all film families to fall at the same location in the graph space.
Does this help?
Incidentlly, since the purpose for establishing a PRSP is to discover the true RELATIVE speeds of various films, you should base the PRSP test on the film/dev combination that you consider to be honestly rated, then use that test exposure (suitably modified to adjust for differences in ISO speed) for all subsequent tests.
If you establish a PRSP for each film/dev you use they'll all appear to be honestly rated (not likely) which really defeats the purpose.
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