Disappointingly, I can't figure out the timestamp as convincingly as I would have hoped.
I was sure there was enough overlapping assassination footage to piece a timeline together but that appears not to be the case.
The frame in Bell showing the limo entering the shadow of the underpass can be timed at approximately 9 seconds after the headshot.
10 seconds later we have this frame from Bell:

Weigman has finished filming the Hesters and is in the process of standing up.
Frustratingly, the Bell film cuts here and resumes showing that Weigman has suddenly arrived near the Newmans:

At the moment I can't bridge this cut.