Cooking the roast at a high temperature initially and then reducing the heat gives it a great sear on the outside and lends to the perfect tender roast. Please keep in mind that the weight and thickness of your roast with greatly affect how long it needs to cook. Thicker roasts will take closer to an hour and roasts that are about an 1 1/2 inches thick will be done after about 15 minutes at 325. For this reason, it is very wise to keep a thermometer in your roast so you can regularly check the temperature at the thickest part of the meat. I love instant read thermometers with a probe that has a long wire, so the digital monitor can stay outside of the oven and you can keep an eye on things without opening the oven over and over.