I would be concerned with there being far too many boxes per page. I am part way ( about 200 ) through this challenge. At the 100 mark I asked the question to my assessor ( on the Patreon plan ) whether I was cramming to many in because I was doing 12 per A4 page. They said yes, so I reduced it to 6.

This allows you to test yourelf with longer straight lines and will test your accuracy and visioning of vanishing points much harder, as well as making assesment easier due to better clarity.