I have heard the number settings explained a bit differently. Rather than being the amount of load required to engage and increase in RPM, they represent the actual level of RPM increase once the system in engaged (as determined by the Letter setting). 1 being the lightest and 3 being the strongest. So in reality the number setting dictates when the RPM increase will end. 1 being a lower RPM increase will remain engaged longer that than 2 or 3.
There is a graphical representation of this that makes this very clear. From studying this I decided to change my ZO setting from B2 to C1. According to the chart this change would provide the results of a quicker engagement (which I prefer) but would remain engaged for a similar duration as B2. (C2 shut down too early for me).
I'll see if I can find the chart, which I think I pulled from a Mapple discussion on ZO settings.