You ought to get your eyes checked. The Z190s occur between the two orange lines above.
There are other ways to relate the Secret Service film to the zfilm. You can time the car as it moves down Elm. You can relate the position to the painted road lane lines to the car position in both films. Or you can develop your own 3D model. In all cases the president is between the lamppost and the Thornton sign at z190 to z200. Using lane lines, in the SS film still that shows JFK visible to the SN, the position of JFK is opposite the end of the of the left lane line dash (third dash from the corner). That lane dash end point is just past a point that is directly opposite the lamppost (ie. on a line from the lamppost perpendicular to the curb). So JFK is between the lamppost and the Thornton sign.
Projecting that onto a scale map of Dealey Plaza as I have done shows that JFK was at that point at z195.
Your lines are not perpendicular to the street curb. The car is following the street direction so the parts of the car that are perpendicular to the car's direction (eg. the front, back, windshield) are perpendicular to the curbs:
Since the road-perpendicular line from the back of JFK's position in the photo crosses the lamp post at a point that is higher above the ground than the line is above the car, this means that the back of JFK is AHEAD of the lamp post. Since it crosses below the foot of the Thornton Freeway sign, it is before that sign. The road-perpendicular line from the front of the car crosses the Thornton sign at about the foot of the sign post, so the front of the car is just behind the Thornton Freeway sign (i.e. the line which passes just above the top of the hood crosses the sign at a point that is lower in relation to the sign). The distance from the front of that car to JFK's position is likely about 10 feet. JFK, therefore, is ahead of the lamp post and at least 10 feet from the sign.