The wheel center line is usually the most convenient point to make the master horizontal datum (0) line. However,sometimes the bottom of the chassis (frame) is also used.

In general there is a vert and horz datum line. This is not always in the same location from car to car. It is want ever point is established a points of reference for a particulate model. It could be the fire wall or front when centers. The remaining grid lines are located relative to those two "zero" lines. When I get home this weekend I will see what I can find.

Generally some on this information can be found in the original master collision manuals.