The challenge completed it´s purpose on you, by the final stages of the exercise i couldn´t point any imperfections in your boxes, which begun less accurate to become really well formed boxes, well done!

There are some pages that contain more than 5-6 boxes, this makes seeing your markings and mistakes harder,try to stick to the instructions. This forced you to draw smaller boxes and i can tell it made it more difficult for you.

I highly recommend pulling the paper from the sketchbook before making any exercise to be able to draw more comfortable.