When you roll a car down a street with a 3.13° slope and re-measure the "mark", the relationship for vertical to horizontal is 1ft/18.3ft.
A 10" change should have resulted in the "mark" moving 10/12 = .833... x 18.3 = 15.25ft farther down Elm St.
The entries for CE884 z161-166 make it quite obvious that a ballistic, as well as speed adjustment was being made for syncing.
The distance from the limo front to JFK within the limo is 15.11ft
The distance between first shot determination(FBI/SS) at Station#3+81.3 and CE884 extant z235(Station#396.
= 15.5 ft = the same distance between StationC - 234.5 (where the measurements started) and directly below the snipers window at Station# 250.0
The "mark" is and always was JFK's physical location within the limo.
Follow the bouncing ball.