Also, what do you mean about the "initial positioning?" Did you see him do something before he hit the guy that was suspicious?
I just watched the video again: BG walks in, looks at money in his hand, looks at the guy at the counter, puts his hands down likes he's waiting, looks at the guy again, then right (all of which appears to be aimless observation while he's waiting to order), and hauls off and clocks him.
Until that point, he did nothing suspicious at all.
I agree with you about making him aware that you see him by saying "how are you." Maybe standing at a different angle facing him a little more. Would that have stopped him, made him spend his money instead of taking yours?