At first I had Perception in there to indeed mirror the formula of HP to some degree. The more I thought about it the less sense that made to me, so I removed it.
I have Sanity defined as:
How much agony and pain a person’s mind can handle is represented by their Sanity score, which is influenced by their Learning and Logic attributes, as those aid to process these situations with rational thoughts:
So narratively that wouldn’t fit too well with Will, but I also made that choice because of concerns of balancing, as I didn’t want to become Will too much of a valuable attribute. Note though that Will helps recovering from Sanity damage, so it still factors into Sanity, which puts Will onto the same level of utility as Toughness, in my opinion.