I assume the answer to this is simpler when it comes to storygames that lean more heavily into the "game" part. This is especially true in RPG-style games where staying alive for as long as possible or reaching some other predetermined goal is key. Same goes for puzzle games and the like.
My question is more directed towards those "story" leaning storygames. What makes you decide how difficult your game is in the 1-8 scale? Is it an estimate of how likely it is the reader gets a "good" ending, or do you go by another method?