Cooking times are only guides. For perfectly cooked steak every time, a meat thermometer is a must.
Medium-rare: 145°F (63°C)
Medium: 158°F (70°C)
Well done: 170°F (77°C)
When meat stands before it is served, the internal temperature rises.
You must therefore remove your meat from the oven when it is at 131°F (55°C) for medium-rare doneness and at 144°F (62°C) for medium doneness.