I think of prestige as a 100 point scale. An F prestige school in DIII would have a zero rating and a two time defending DI champ being a 100. Each partial letter grade has some sort of value, say a "B+" prestege in DIII is 30-33 (just kinda making #'s up). WIS does it's mumbojumbo equation and your program comes out as a 33.2, it then looks at the chart and poof, you get an A-.
Now, I also think each recruit has some "prestige value" threshold. For example, say a recruit is a low level DI guard. The recruit will have some sort of numeric value that says something like:
Prestige values > 50 can recruit this player straight up (so the player will appear on the DII screen, if you're a DII program with a prestige value over 50)
The player will drop down to teams with a prestige value between 44 and 50 (in increments, he'll drop to higher prestige values first)
The player can be pulled down by prestige values between 40 and 50
I think there is some sort of formula that is used to create the players prestige value variable and each player has a real number value.