How would one go about finding the depth in these examples?


I am studying parallel projection. In this example all the depth lines are true 45 degree lines. How can I find the correct depth for the side planes that will be equal to the front plane? (a cube)
In linear perspective the 45 MP solves this. What about in freehand perspective? Is there a way to objectively know this or you have to pick an arbitrary point as part of the "creative license"

Any help would be great appreciated. I am frustrated here.