In my opinion one punch KO should only happen when punching the back of the head. While hitting someone in the face with can of beans would be very dangerous, it's not working really well for gameplays sake.
For me it's not really realistic to get KO'd when getting punched directly to the face once. Normally the one who is about to get punched gets to brace for the impact and/or move away from the punch, making the punch less effective. But we can't do that in DayZ.
Surprise hits to the side/back of the head should be more guaranteed KO. That's how people get knocked out in real life too.
Last time I saw someone get punched in the face he only looked angry. And didn't even retaliate.