One last comment and I'm done, and it's only because I love Prime Rib! Please don't cook it at 350 degrees. If it's a whole boneless prime rib in the 14 pound range, or a whole bone-in prime rib in the 18 pound range your carryover cooking will kill you at 350 degrees. I call it culinary inertia. Think of a 16 pound bowling ball, push it gently and it will come to a gentle stop. Throw it as hard as you can and it will travel a long way.
A roast cooked at 350 degrees to say 130 degrees internal will continue to cook another 10 to 12 degrees AFTER you pull it out of the smoker. A roast cooked at 200-225 degrees will continue to come up maybe 2 degrees AFTER you pull it. Cook it gently, and if you want more color throw it on a hot grill, or in a pre-heated 500 degree oven for five minutes. You don't want to spend $$$$$ and overcook your meat.