Knowledge (Forbidden) & Sanity

[Mediator9292]

In my playthrough of the Strange Aeons AP, we're using the Sanity system, and to make the knowledge the PCs are gaining feel more impactful and dangerous, I'm trying to make a Knowledge (Mythos) skill similar to that seen in the Call of Cthulhu/Delta Green systems, and also present in the variant d20 Sanity system.

Mechanically, I'm trying to make an eval script that codes so that every rank in Knowledge (Mythos) (id skKnoMyth) results in a -1 reduction/penalty to either the Sanity (tSanity) or the Displayed Sanity (tDispSan). The problem is that I have no clue to to do something like that.
Any advice on how to at least get started?