I'm not really sure about skill level equations and the like but I don't think there's a universal formula that could be applied to every single skill. I could be wrong though, so I suggest having a look on the
Guild Wiki Skills page. It lists every skill and how each is affected by every attribute level.
Or you could head on over to GWGuru's very own Frog Devourer's thread
here. He's done some incredible work with skill cards and they are, imho, much prettier to look at than Wiki
Hope you find what you need.