Remove roast from fridge. Pat dry. Let stand 1 1/2 hr. at room temperature. Meanwhile, preheat oven to 250°C (500°F). In small bowl, mix flour, salt and pepper. Rub roast with mixture (if coating smaller than 8-lb/3.5-kg roast, there may be leftover rub). Place prime rib bone-side down in roasting pan fitted with rack.
The prime rib is roasted at high heat then, without opening the door, the heat is turned off and the roast remains in the oven for 2 hr. To calculate roast time for assorted sizes of prime rib, for every 1 lb (500 g) in weight, multiply by 7. This equals the number of min. roasted at 250°C (500°F). After this high-heat cook, the heat is turned off (oven door is closed at all times) and the roast stays in the oven another 2 hr. The 2 hr. is a fixed duration for any size roast. Example: An 8-lb (3.5-kg) prime rib roast cooks for 56 min. at 250°C (500°F). Then the oven is turned off and the roast stays in the oven another 2 hr.
The oven’s retained heat cooks the meat while keeping the centre pink and tender. Check the roast for doneness. If more doneness is desired, turn oven to 180°C (350°F). Place roast back into oven to cook. An instant-read thermometer inserted into centre registers 63°C (145°F) for medium-rare.
Transfer roast to serving platter or cutting board. No resting is required. Reserve pan drippings to make Classic Pan Gravy for Prime Rib, if desired. Slice roast and serve.