Weee! Perspective!
Cedricgo was almost spot on, but you cant use that vertical line as the back corner for any square that is not equidistant from the two vanishing points and vertical to the vanishing plane.
I wanted to really get how this works down, so here it is. As noted, this works best within the bounding plane of the vanishing points, I know that there is math that you can do when you are out of the bounding plane to make it work as well, however it is probably easier to do this part and then use the "Fence" technique that Marc outlined in Term2 Perspective 2.
But it also might just be better to eyeball it and go with what looks mostly right. You only need to do this crazy way for architecture.
I hope this is visible, I am uploading a file that is about 2k pixels wide.
... Yep! Just posted and checked. Click on the image once and it will come up all small, click on it again and you will have the full version flooding your screen so you can see and save it.
Hope this helps! MATH!