The 30", 60" and 90" measurements are to the top of the horizontal members.
As far as the height being wrong, it appears that the field drawings call out for more than a 6" standoff from the top of the pyramid. I think if you make the top of the disc the 120" mark with a 20" (reflective) pole, you should be able to make the standoffs the correct height.
Also note the diference in the amount of chain on the field vs team drawings. I would also suggest using 2"x2" (or slightly smaller) for the pole instead of the 2"x4".
**Disclaimer: I only looked at the drawing for a few minutes.