I'm making a rib roast for X-Mas and after looking at numerous cooking books and mags I'm confused about what degrees registering on the meat thermometer (I have an instant read) will give me a medium rare roast. I've seen the range listed from 125 degrees up to 140 degrees. Help! I don't want to wind up with too rare meat (which a lot of the relatives won't like) or well-done (which will be awful). Please advise.