The curve I described has a time constant. In electronics, it is RC or 1/RC. A developer has a similar constant. The maximum contrast index used to be called "Gamma infinity" and was a theoretical asymptote of the curve of contrast index vs time of development. The maximum contrast of HP5+ in Rodinal is about 0.65, which is approached quite rapidly in the 1+25 dilution, and never reached in the 1+50 dilution.