How long to cook bone in prime rib per lb at a low temp?

The author is preparing an 11-pound, bone-in prime rib roast for cooking. The roast will be dry-brined with koji rice for three days in the refrigerator. After brining, the weight is estimated to be around 10 pounds. The author is seeking an estimated cooking time to achieve a medium-rare to medium doneness. They plan to use a reverse sear method: slow roasting followed by broiling. They are using a wireless probe thermometer and intend to cook to temperature, not time. The author found inconsistent cooking time estimates online, creating uncertainty. They considered cutting the roast in half to reduce cooking time. The cooking temperature will be 250°F. They intend to rest the roast for 20 minutes after cooking. The primary question is how to estimate the start time to have the roast ready for dinner.