Because I use a meat thermometer, I sort of play it by ear...that's how I do a lot of my cooking honestly. However, the best method is to originally put it in a 500 degree oven after letting it sit out at room temperature for at least an hour if not longer. I pat it dry with paper towels, rub mine down with oil, then salt/pepper/season it, add the rosemary and garlic. You let it sear on the outside in that piping hot oven at 500 degrees for anywhere from 20-30 minutes. Then you can either TURN OFF the oven or, what I do, is drop it down to 275 and keep an eye on the meat thermometer but depending on the . It depends on how rare you or your guests like it, my daughter and husband likes it rare, I'm more of a medium to medium well and the other kids like it around that too. The outer portions will be slightly more cooked than those at the center.

Here's a sort of similar method but has a different cooking temp of 325 - [link to www.simplyrecipes.com]

You can use the drippings to make au jus after.