we know that "b" and "a" are proportionate. How do you find "c" in the other location? Also, is it possible to find proportionately correct shapes in any location within a scene? by "proportionately correct", i mean a shape following the same length, height and width in any location within that piece. (for example, the example below, but say above the horizon etc)

Weirdly enough, i know how to find proportionately correct shapes in 3-point perspective, but this one is just challenging.

>inb4 just use intuition
I do. I just want to know the math/geometric logic behind it so i can get a deeper understanding on it. Any books or references that talks about proportions in perspective or perspective in general is much appreciated.