Your tires are a little wider and taller than the spec for 986 - I bought a set of Cayman wheels with big tires on them, fronts 245/40/18 and 285/35/18 and the fronts were too tall and didn't clear the strut perch. (Folks with something like pss9 would have been fine but not me with my stock struts) No amount of reasonable offset would have fixed it. At 235/40/18 I bet you are very close, but if you could successfully mount the tire/wheel you should be good
I don't know about the offsets though, that is like voodoo to me.