At this point, I am wrestling with saving throws. As best as I can tell, there are two types of saving throws: those intended as general threats, and those intended to put the fear of God into the party's tank. In other words, is the save intended to challenge all characters on only those character likely to interact with the effect?
Using those two concepts, I built "Character Save + 14" equations in order to generate DCs for those two scenarios, and I am getting very close to reproducing Monster Manual I numbers. That pleases me. There's more refinement to do, but I believe that I now have the correct logic behind the numbers.