6–8 for medium and 10 for well done. To check if your steak is cooked how you like it, temperature probes can be very helpful. Rare will be 52C, medium rare 55C, medium 60C and well done 71C.
and this represents how a medium-cooked steak should feel. As you stretch your thumb to reach the ring finger, now you've got medium well, and the tension in the thumb when touched with the pinkie ...