Well, to add to my previous post, many developing agent combinations are superadditive and also are influenced by reaction with sulfite, so just assigning a figure to Metol-Hydroquinone of 1.9 is misleading as the ratio of the two is important as is the ratio of HQ to Sulfite as is the ratio and absolute values vs pH in the range from 9.5 to about 11 which covers low to high contrast developers.

This, I guess, led me to the conclusion that a single number is simply insufficient. The tables or graphs can be seen to change slope for different developers, and I posted a sample table in another thread. Given a "starting point" can waste a lot of film, but given the table can just about lead you to the right value immediately.

In addition, please note that on the time (X) axis, the values are logarithmic which means that the data is non-linear and this is certainly not shown in the use of single values, nor can it be shown.