What Tex_77 told you is spot on. I normally set temp at 250 whether smoking or by oven. Get yourself a good thermometer. Definitely pull at 120 and set oven very high to sear, and pull it out and loosely wrap with foil. Let rest at least 20 minutes. Remember temp will rise at least another 5 deg searing and 5 deg resting. Usually finished temp is 128-131 for medium rare. No need to inject prime rib. I always put a rub on 2 days prior to cooking it.