/ic/ - Artwork/Critique

>>6815021 continued:
I am now attempting to do the same thing but using a grid to help with the orientation of the cubes.

While construcng the grid I ran into a problem. I understand that I must use a measuring point on the horizon to establish the lines of the grid that are parallel with the horizon, but I don't know where on the horizon the measuring point is meant to be placed.

>all measuring points of perpendicular planes will fall in a perpendicular line through vanishing points of those planes
>all measuring points of planes parallel to ground plane will be located on horizon
>rectangle of first space is optional - to repeat simply take diagonal of the first space to the horizon or to the perpendicular thru vanishing point and back
>the depth of the first square is optional - establish measuring line by drawing diagonal thru square 8 where it cuts other division lines, marks off the depths of other squares

There is no text that describes where measuring points and (vanishing points) should actually be placed on the horizon line. What is the method for determining these points? What is meant by "optional" in this context?