Helo new here I'm in a bit of a pickle need some advice. I've always cooked holiday meals for my family and have always gone the traditional Mexican route Tamales for Christmas. This year I want to surprise my family and make a prime rib roast . I bought one last night it's 13 pounds 5 bone , bones have been removed and tied back on . I have been reading a lot and I'm on information overload so I'm starting to loose my confidence here please help ? Do I cut I half or keep it whole and what's best ? High 450 for 15 minutes then lower it 325 and how long would a roast like this take ???