I don't remember where I heard about a 45-45-45 way of cooking prime rib, but I do know it made a mess in my oven. I was told to cook my prime rib at 450˚ for 45 minutes, then 350˚ for 45 minutes and then turn the oven off and tent my prime rib with foil - loosely for another 45 minutes - outside the oven. So next time, I made poor John (my husband) go out in the cold and turn on the grill. We cooked our prime rib for 45 minutes at 450˚ and then brought the meat in to the kitchen oven and cooked it for 45 minutes at 350˚ and then checked it with our handy dandy meat thermometer - it registered about 135˚. Then, we tented it loosely with foil (outside of the oven) for another 45 minutes. After that it registered about 145˚ and believe me it was wonderful! And, my oven was clean, which made me very happy :) ! We like our beef cooked to med. rare so if you like it more done, be sure you use that meat thermometer and if you like it a little more done, turn off the oven after the 45 minutes at 350˚, tent with foil loosely but leave it in the turned off, but warm oven. This seems to work on different sizes of roasts that I have made. The center cuts are more rare but if you want more done, simply slice your meat and put it back in the oven to keep it warm. Better to have a more rare prime rib than one that is too done - in my opinion. I hope this works for you. OK, here's just a bit of extra info for you. When you purchase your rib roast, try to get the butcher to cut it off the bone or just use a sharp knife and do it yourself at home. Season your roast with your favorite meat seasoning. I like to rub in a mixture of salt, pepper and crushed garlic. When you put it on the outside grill for the first 45 minutes, John suggests you put it on a wire rack, with the meat loosely sitting on the bones. If you put this into one of those throw-away aluminum pans, you will avoid the possibility of flame ups from the dripping fat. Add a little water to the bottom of that pan and you will be able to save the juice for au jus sauce to serve along side the slices.
45 minutes on the grill at 450˚, 45 minutes in the oven at 350˚. Turn the oven off. Tent the Prime Rib loosely with foil and let it sit on the counter for 45 minutes. Works for me!