7:28 AM, Saturday May 23rd 2020
I'm only at box 30, but from what I understand it's about grasping the relationship between parallel lines. It's not specifically about finding the vanishing point, but instead to read the parallel lines and then have an intuition about where that vp wil be. Then it's balancing that relationship so that all three pairs of parallel lines respect the different vps of the box.
The closest I get to graphing a vp is to ghost in a line that converges with its parallel twin, then dropping a dot at the approximate point where I think the perpendicular line would meet it. I suspect anything giving more guidance than eyeballing it like this will defeat the purpose of the exercise. Afterwards, I look at the plotted lines and spot where I got stuff wrong, then use that knowledge to second-guess my choices when I do the next page.
I don't know if that helps your question, but thanks for the topic! I do sometimes wonder if I'm doing this right.