11:41 PM, Tuesday August 4th 2020
I'm glad you followed though with what I asked! I can see nice improvement just form these pages so here's some further notes:
With the back corner issue, which is pretty common, it happens because of the small mistakes from the rest of the box adding up and the result is the back corner.
This method of constructing boxes helps some people avoid the back corner issue and thinking of your parallel lines in relation to eachother and not just their relation to the vanishing point as illustrated here will also improve your construction.
Next Steps:
Keep practicing making boxes in your warmups so you can continue to progress in skill. I see you've already completed lesson 2, so goodluck with the next step in your journey after that!