That would be necessary if you are going to pontificate about Connally's exact position at the time, yes.
Looking at Z-180 (or so) through Z-224 (or so), isn't it fair to say that JBC's head and upper torso were still turned to his right when JFK was struck?
For purposes of trying to determine whether or not the bullet that passed through JFK's lower neck also penetrated JBC's back, is it necessary to know in which Zapruder frame it allegedly did that?
Does one's inability to say with absolute certainty which frame that is (possibly due to JFK's and/or JBC's being hidden from view behind the Stemmons Freeway sign and/or the possibility that they reacted differently to being shot) preclude its having happened?