Yeah, the pics (drawings) in the AIM are deceiving, but I bet if you had a way of measuring from the firewall, it would be 114.25 " back to the antenna. Section 11-13 shows the reference points where measurements are taken.
There are two ways to measure from the firewall (Zero starting point for the PV).
1- No windshield - Use a piece of thin wood (a yard stick is good) held flat against the firewall and with someone holding the end of the ruler, pull it across the dash to the stated PV area to get the back depth. Then perpendicular that to get the location on the quarter.
2- With a windshield installed - use a wood or plastic yard stick placed flat against the firewall and stick out the side (about 45 degrees) of the hood hinge area and measure from that point to the PV target area.
The short PV from the center of the car is basically 1/2 the distance between the two quarter gully centers.
That's how I verified my mounts location recently. Back in the day I just used the measurement from the back of the quarter edge of the original that was removed.
Mike