I think 51-52's should be called Bug-Catchers, you can easily spend an hour trying to get all the bugs out from around the headlight buckets after even a short night drive.
Dimension "B" is explained below. Basically it is to relate the headlight center to a 30" reference line drawn on the wall.
If you use your method, Dave, on a truck that sits taller, you end up with the headlights aimed too low; draw a line from the center of headlight to the point you are using as a reference, then draw a line from a headlight that starts out 4" higher to and beyond the same point on the wall. The taller vehicle's headlights will hit the ground sooner.