After reading countless posts, articles and books, I cooked my first prime rib tonight. It didn't go well, by my standards. For a first timer, there is a considerable amount of mystery (and cost) involved in roasting a prime rib. Sourcing the cut of meat is the first hurdle. There are numerable good to excellent butchers in NYC, and choice is not easy. I eventually picked Florence Meat Market, and it was an excellent choice. They are very accommodating, and cut my meat right in front of me. Quite a delight to see a well butchered cut.
After weighing the many different ways of cooking the meat, I decided on the high heat/ medium heat option: 450F for 30 minutes/and then 325 for there on out. Sounded like a good idea, but the temperature readings were erratic. The initial browning worked well enough (although I would prefer a browner crust--I would try an a stove top browning next time). But the remainder of the time proved mysterious to me. I had a remote thermometer which I used, but I didn't quite trust it. A rib roast (like my 4 rib) is larger on loin end than the shoulder end, so I worried about reliability of the thermometer reading. I stuck it on the small (loin) end, ]I checked the roast and it read 118F. I checked the large end, it was at least 10F lower. Not wanting super rare meat (stupidly, as it turned out), I stuck it back in the oven. When that read 118, I took it out. 118F is below the medium rare I wanted. However, I let it rest for about 1 hour, but probably more (not intentionally, but that's how the dinner went) and the final temperature was about 136F! Too well done for my taste. It was still pretty medium rare in the middle but the remainder proved over cooked. From all that I had read, I had not expected an 18F degree rise during resting. I had brought the roast to room temperature for 4 hours, so that should have been alright.
The best I can see it is to take the roast out at 110F (read on the small loin end) perhaps even lower, to make sure the roast is well cooked (rare to medium rare). Otherwise, one is risking (mini) disaster. In the end, I would much prefer a porterhouse or individually cut rib eyes. For the purposes of posterity, the roast started was 13 lbs, aged 3 1/2 weeks, left to room temperature for 4 hours, and seasoned 1.5 hours before roasting.
Roasting takes practice evidently, and the oven I was using (not my own) proved erratic (get an oven thermometer to ensure readings!) For a first timer, my advice is better safe than sorry: take it out on the rare side, for you can always cook it more, but you can't fix an overcooked piece of meat. I learned that the hard way.