The box will look weird if all three vanishing points are really close. And the closer one vanishing point is, the farther the other two tend to get. If you want to have really dramatic foreshortening your box will look more and more like a 1-point perspective box. If you put all three vanishing points close. About the far edge of a box vs it's opposite edge, that's not exactly how it works. I can't really explain it well in words so try to pay more attention to other boxes people have drawn/ those that show up in examples, and compare dimensions of opposite edges of boxes, making sure you do that for each edge.
apokb in the post "problems drawing boxes with dramatic foreshortening"
2022-08-07 23:08
The box will look weird if all three vanishing points are really close. And the closer one vanishing point is, the farther the other two tend to get. If you want to have really dramatic foreshortening your box will look more and more like a 1-point perspective box. If you put all three vanishing points close. About the far edge of a box vs it's opposite edge, that's not exactly how it works. I can't really explain it well in words so try to pay more attention to other boxes people have drawn/ those that show up in examples, and compare dimensions of opposite edges of boxes, making sure you do that for each edge.