Roasting beef causes the muscles to contract and expel the juices in the meat. It's best to cook your roast beef to an internal temperature of around 130-1355℉ for medium-rare. The meat will be the most tender at this temperature and will retain more juices. Letting the meat rest for 20-30 minutes after cooking is critical. This will give the muscles time to relax, which helps to redistribute the juices in the meat, giving a more tender result. The internal temperature will continue to rise another 5-10°F while it rests due to carryover cooking, so remove the meat before you reach the final desired temperature.