It is odd. You would think that SW would put that info out instead of getting tons of guns getting sent back and having to pay for shipping for nothingThe plate will set the sight up a few thousandths and should help with your POA/POI issue.
What I'm trying to figure out is the angle you mentioned measuring and it sounds as if it's by design. If so I'd consider that a pretty large faux pas from S&W to cut it for a sight that's not anywhere near mainstream. If that's the case...