Modify Element Rate by Variable

I'm trying to make some skills that decrease the element rate by a character's parameter, in the same way that they're used to modify damage. I.E.: increase physical resistance by a percentage equal to the character's Strength. There's actually a lot of skill ideas I want to use that involve using parameters as variables. I have a suspicion that there's some way to do this through the formula bar, but I'm not sure how to do that or where to look.

If you're modifying them via features, this might be helpful.