74 frame dimension help

[E5 Charger]

I have the frame dimension diagram from my 1974 manual (shown below), but they don't explain exactly where you are taking the measurements from. I'm assuming for some of them you hang the plumb-bob from the middle of the 1" holes in the frame rail.  Has anyone successfully done this and can they please explain what points of reference they used? Photos of "L", "E", etc would be awesome.