Right. Well the bottom line is that if you hit the jaw anywhere hard enough it might well break. Too high is less effective as the impact is likely to be spread onto the cheakbone (which could in itself smash),too low nearer the point of the jaw and the impact is likely to be spent in sliding off and across the jaw to ensure a break. That leave the spot right in the middle at the bottom of the curve which to my mind is plumb. Two additional factors should be noted. Firstly, the size of your fist is likely to mean that a relatively large area is hit anyway, so specific targeting is a bit moot and secondly, when the mouth is open you are much more likely to break the jaw, often around the hinge area.
Moving onto the issue of KO. This is a much more elusive commodity than people think and ensuring that you achieve one is not always easy. The aim as you may know is to achieve "brain shake" whereby the brain, which is suspended in fluid in the skull is made to collide with the interior of the skull. This causes neural overload and KO results. The best way to bring about this state of affairs is to strike as near to the point of the jaw as possible without expending your power by sliding off. It should be noted that strikes from the side (hooks etc), to the jaw tend to make the opponent fall in the same direction as that from which the hit came. This may result in a head connecting nastily with a hard object such as the pavement. A harder but more skilled strike is over the top and down onto the jaw which can cause the opponent to virualy sit down, thus lestening the chance of subsidiary head injury. Hope this helps.