Go to http://www.cooksillustrated.com/
and register for the two week free trial which will give you access to the Perfect Prime Rib Recipe
. (I've tried it and it's pretty darn good.)
Better yet, check out the book which tells you exactly what is happening to the tissues at what temperatures so you can make the ultimate informed decision. It also tells you which cut is the best.
The meat will continue to cook and the temperature will continue to rise once you pull the meat from the oven. Use a probe thermometer. Pull at 120 for rare and 130 for medium rare. Keep in mind that the meat on the ends will be more done that the meat in the middle.