Roast: Roast at 425°F for 20 minutes. Then reduce the heat to 300°F and roast an additional 40 minutes (for a 6 pound boneless roast) to an hour (for a 6 pound bone-in roast). If you are cooking a roast bone-in, the bone will act as an insulator and will require a longer cooking time than a boneless roast.
In general estimate 10-13 minutes per pound for total cooking time (for rare), including that first 20 minutes at high heat. (Assuming you let the roast sit out for an hour or two before putting it in the oven. If it’s right out of the fridge, it will take longer to cook.)
Note that the method of cooking directly on the oven rack will mimic a convection oven and the cooking time/oven temp needed will be less than you would need if you cooked the roast on a rack in a roasting pan. If you are cooking the roast directly in a roasting pan, rack or not, start the roast at 450°F and after 20 minutes reduce the heat to 325°F.
Also, the shape of the roast will have an impact on the cooking time. Our roast was rather long and thin, so it cooked up fairly quickly. A thicker roast may take longer than expected.