With rack on the bottom 1/3 of the oven, preheat oven to 500°F.
Insert thermometer into thickest part of the prime rib.
Place roasting pan on the oven rack and set timer for 15 minutes.
REDUCE HEAT to 325°F.
Determine the preferred doneness and calculate cook time based on the weight of the roast. (See chart in post.)
Monitor the meat thermometer. Remove roast from the oven when temperature is about 5-10 degrees below desired doneness temperature. Cook for a little longer, if needed, to reach that temperature.
Tent with foil for 30 minutes before slicing otherwise it will lose juices and be tough. Once tented, temperature should rise another 5-10 degrees and reach final desired level of doneness.