Determine when a steak is done, by comparing the firmness of the fleshy part of the hand at the base of the thumb to the flesh of the steak. 
 
Take the forefinger and just touch it to the tip of the left thumb. Don't exert any pressure, the lightest touch is correct.

Take the left forefinger and poke the fleshy part of the hand at the base of the left thumb. The feeling you get is the feeling of a rare steak. Now touch the steak. For medium use the second finger of your left hand to touch the fleshy part of the thumb. For well done use the third finger of your left hand. Voilà... perfectly done steaks every time.