For the first 50, I noticed instances of arcing, like in box 10 and box 34. I also saw some lines that appear have been drawn over multiple times, like box 6 and box 44. While your convergences show decent accuracy overall, I noticed lines pair off and converge towards two vanishing points, such as box 12's red lines, box 32's blue lines, and box 49's red lines.

For the next 50, I see improvement in the quality of your lines, with less arcing overall. However, I still see instances where a line was redrawn, like box 54 and box 79. Your convergences also show improvement, though there are still instances of lines pairing off like box 57's red lines, box 74's red lines, and box 90's blue lines.

For the last 150, your lines show continued improvement, but there are still instances of lines being redrawn, like box 115, box 140, and box 215. For this challenge, it's better to draw one smooth, confident line than to try to redraw a line multiple times.

Your convergences also show improvement, though there are still instances of lines pairing off such as box 128's red lines, box 182's blue lines, and box 221's red lines. In these boxes, the inner lines (green and orange in this diagram) diverge instead of converging at the vanishing point. Keeping in mind the angles of neighboring lines can help when planning the box's construction.

This also ties in to your comment about the rear lines veering off-course. Little mistakes build up throughout the box's construction, and placement of the final corner/edge is where they become most apparent. Don't worry about trying to make that last edge come out perfect. Focus on practicing the skills and techniques this challenge is trying to hone.

As an aside, some of your boxes get a bit small on the pages, like box 3, box 106, and box 147. While I saw fewer small boxes towards the end, drawing larger boxes is more effective for practice, as it'll help you engage your whole arm from the shoulder, and will engage more of the part of your brain that processes spatial reasoning.

Also, box 54 has line extensions going in the wrong direction. Box 88 is missing some line extensions. It's not a major issue since I didn't see many errors, but I want to point it out for future reference.

All said, I'm going to mark this challenge as complete.