Let's say a door has a ratio of 3 to 1, height over width. How do we accurately represent that in perspective. The door on the back wall is easy, but the ones on the side walls, did you just guess at how wide to make them? I run in to problems like this when drawing comics. Sometimes it doesn't matter, but sometimes I realize an incorrectly sized window or hall or cubicle partition massively throws off the scale of a scene.
