I always do a rib of beef for Christmas ( and yes, usually from Costco) and I do it simply, leave it out for a few hours before so it doesn't go into the oven cold, sit it on a rack in a roasting tin with some onions, veggies, etc underneath (during cooking I often put some water so the veggies don't get burned and this helps to make good gravy) anyway, I roast it as per the time per kilo deal, however, because I never trust myself, I always take it out about 30 minutes before the timer goes so its well done on the ends, and med rare in the middle.
I have a digital meat thermometer, however I just try to trust my judgement as the meat thermometers always seem to overcook the meat for me. I touch it and stick the thermometer probe in so I can see if its hot in the inside (just to the touch)
And, when I make my roast potatoes and my yorkies, I ONLY use beef dripping...over the top, but oh so good!!!
Good luck!!