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Thank you for your reply Phil. Please correct me if I'm mistaken in the following. The current approach is to assess the EFS of a film at different development times by relating the speed point on its characteristic curves with that of a theoretical function (ANSI ISO 6-1993) whose speed point is as reported by the manufacturer (as I have been doing for years with the ISO triangle method I learned from your book). As you mention, a weakness of this approach is that the theoretical function used as a standard may not even exist, let alone accurately reflect the film's "true" characteristics. The PRSP renders this process more accurate by relating the speed point of individual curves with that of the speed point for a film that is presumably more accurately rated by the manufacturer. The EFS's obtained with the PRSP should be more accurate if the exposure is properly calibrated, and assuming that the particular film/developer combination used as a benchmark renders a speed as reported by the manufacturer.
My thinking in the previous post was that if we could calibrate the exposure axis in absolute terms (with a relatively simple procedure), and then read off the curve's speed point absolute exposure directly (as you suggest by consulting the look-up table in ANSI ISO 6-1993), then we would not need to have any other standard for comparison at all. This EFS would relate directly to the one we dial into our meters. In fact, I think that even the manufacturers reported speed would be irrelevant in practice. So then, our EFS/SBR chart would contain absolute film speeds on the x-axis, rather than relative ones based on a more or less accurate reference point. Do you think that this approach is feasible?
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