The key is a meat thermometer and paying attention to it. It is better to check the temperature of the meat often rather than risk overcooking it. I'm on the high heat to sear and then low and slow. Once you take it out of the oven, it is important to let it rest, about 20 min if I recall right, otherwise the juices will flow out when you cut it. While it is resting, the heat in the roast will continue to cook it, so it is best to take it out when it's not quite as done as you want it. It won't cook as quickly out of the oven, so it you wanted it to rest for 20 minutes, you'd take it out needing say another 10-15 minutes of cooking time. If all else fails, it is much easier to cook it a bit more than to uncook it, so it is better to err on the side of underdone.