I typically will tighten that little nut right up until the point that the wrench begins to ever so slightly move the metal on the corners. I don't know the torque value for this, and it's very much a "feel" type thing. "Snug". YMMV, proceed with caution.
Just giving everyone an update. Picked up my Benelli from my FFL yesterday and the damaged stock has been replaced. I was starting to get worried because I called customer service last week to check on the status and they stated they did not have a record of it. My FFL called them and they stated the same thing. It's been a long two months and dealing with Benelli's customer service has been a real pain but I'm glad they did the right thing.