I pat the room temperature roast dry, season with pepper and put into a 450F oven to crisp the outsides. Turn down to 325F after 15 minutes and roast approx 15 minutes per pound and test with a thermometer as Pierino says. Do not let it get above 130F, and rest as noted above.
Bone in versus no bone will alter times so use the thermometer regularly.

There is no proper time per pound. What you actually need is good instant read probe, say a Thermapen. Indispenable. For medium rare you can finish when it tests for 130F. Then you let it rest for 15 minutes tented with foil. This is so the juices that are bubbling in the center of the cut work their way back out. I don't care what anyone say's, I don't trust the watch the clock and finger poke method. It's all about internal temperatures. It's also unlikely that your oven is calibrated exactly as mine no matter what you set the dial for.