I think that your chart helps. The other way to illustrate it verbally
is to posit a "theoretical pyro stain" of 50% increase in effective
density over the silver alone. If a negative ran the scale of silver
density from .2 to 1.6, it would have a range of 1.4--increase the
effective densities all by 50% with stain, and the range becomes .3 to
2.4, for a range of 2.1--much more contrasty.
My first look at actual PMK pyro shows an increase of less than 50%, and
it doesn't appear to be truly linear, but it sure does increase both
overall contrast and close value separation with HP5+ compared to
standard developers (when printing with ferric sensitizer). I haven't
been thrilled with the tonality of FP4+ with standard developers, but
haven't yet tried it in pyro, or Terry's amidol either. I'll look
forward to seeing how they work.---Carl