Hey, Charles,
I'm not able at the moment to view better copies of the Hughes film, so I'll take your word for it that Atkins looks at his watch in the clip you've posted.
What I know about this segment is that it does cover the instant when the first shot shot was fired, at Z160 plus-or-minus. McLain on his cycle has just passed out of the frame, and the walking cop (D.V.Harkness) is still in the frame, when the first shot happens. So you are good on your timing there.
The Zapruder film actually shows the 1st camera car on Houston Street in a few frames, between the people lining the sidewalk. At Z230, after the first two shots, the car with Atkins has passed the alley between the Criminal Courts and Records buildings. I think that works with your timing as well. I'm pretty sure you have to have the absolute full Z frames to see the motorcade on Houston, since obviously Zapruder's focus was the limousine, but I know he filmed the cars moving slowly along DURING THE SHOTS. If I remember right you can see the red sun visors and convertible top cover on the Mayor's Comet, the greenhouse of the next car, which was a chevy sedan with the "pool" reporters, and the yellow first camera car...not sure what color the second camera car was.
To me, these little details are what assassination research is all about, and I think you might be the discoverer of the Atkins watch look.
Thanks for a cool thread,
Sim