I was taught that when you are in a real life scenario, If you have a clear shot at the attacker, it is 2 center mass and you have a split second to make your judgment if he/her is still a threat or not and if they are, you go for a head shot. Now that brings me to my range exercise that i read. (guns and weapons magazine is my BF and has many real life scenarios) The range exercise explained is to pull out from your holster, 2 center mass (chest shots) and a head shot. It is said that if you accomplish this exercise in under 2 seconds, you are pretty good. Experts average 1.5 seconds. Also there is a great article on re-holstering. It talks about how it is just as important as any other part of the gun fight and safety routines. Anyways that is my 2cents.