Clint Hill actually sees JFK "grab at himself" after the first shot.
He saw the moment JFK's arms flew up and he slumped to the left.
In the "When Was JFK Hit?" thread I demonstrated that the first moment we see the beginnings of this reaction in the Z-film is z225.
It is a neuromuscular/reflex reaction to a shot passing through him at z222/223.
Altgens 6 shows Hill apparently looking directly at JFK as he begins to slump to the left. This picture is taken about one and three quarter seconds after the first shot. In three seconds from this moment JFK will be hit in the head as Hill tries to get onto the limo.
Hill is fully aware of the first shot and JFK being hit by it (the idea that Hill and all the other Secret Service agents on board just ignore a missed first shot is too silly to contemplate).
He is about to react to it.
So, let's forget about first shots at z160 or earlier because we would have to believe that Hill (in Altgens 6) stood there gawking at JFK for over five seconds at least.
Even in Andrew's scenario of z190, Hill stands there for over three and a half seconds and still shows no sign of reacting.
It's an eternity in such an intense situation.
When shots are fired people react very quickly.

Landis, Hickey and Ready have all spun around to their right and rear as a result of the first shot, just as they reported doing so.
Hill is staring at JFK who is reacting to being shot.
Was the first shot 10 seconds before this pic was taken, as Graves suggests? [

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3.5 seconds, as Andrew suggest?
Or 1.75 seconds, as I suggest?