My father, Bill Smith (or Dr. Wm. Stanley Smith, as many knew him) was a great grillmaster, and he turned out a spectacular burger. When I was about 45 he finally gave me his secrets:
70% lean ground chuck
5 burgers = 2 lb (or 6.4 oz each)
Tonight I made a couple of beauties: medium rare, juicy and good. It's raining out so they were cooked on the indoor gas grill (that sound you hear right there is my father turning in his grave: only Kingsford charcoal briquettes ever burned beneath his burgers), 5 minutes on one side and about 2-1/2 minutes on the other. Of course, the timing varies according to your fire, how far the burgers are from the fire, and how done you want them.