From The River Cottage Meat Book - "However, it seems to me that it isn't just your guests who are honored by a roast. So is the animal who has died to provide it...choosing a prime cut and roasting it seems to me a complimentary gesture. It's like saying, "We may boil your shins in wine and make your kidneys into pies, but this fine, lean loin, with its chine and ribs and beautifully marbled meat, we will keep this whole, season it only with salt and pepper, roast it hot, serve it bloody, and think of you!"
1. Shape and size depending, roast for 25-40 minutes (preheat oven) @ 425-450 (judge with a visual)
when done open the door to the oven (30-60 seconds) to cool down to 325
2. He goes with 325 (rather than the conventional 350) for the slow cook after roasting
watching for an Internal temp of 125 = Rare
3. REST THE MEAT at room temp or slightly above FOR NO LESS THAN A HALF HOUR cover with foil
He also highly recommends placing the roast in a roasting pan, not elevated. If there's a such thing "extra" fat can be poured off as necessary.
Rough guess as to time would be 8-9 minutes per pound but SINCE YOUR CUT IS 12 POUNDS use the thermometer. This method has always worked for me. Cheers, j