Hi Bach I'll be handling the critique for your 250 box challenge but first of all let me congratulate upon completing it, it certainly requires time and patience.

So now let's move to the boxes.

-One consistent problem throughout the majority of your work is the tendency to extend your lines in the wrong direction, keep in mind that your initial Y points to all vanishing points, so when extending you should be starting from your initial dot and move where the lines are pointing towards. Take a look at this rough diagram. It seems that you corrected it as you went but it took you some time to do it.

-Now another problem that shows up less frequently is lines converging in pairs as shown here, it is important that your lines converge in sets.

-It seems that you consistently drew small, you want to keep the boxes to a size that's comfortable for the shoulder to draw, and this will make it easier to ghost your lines.

The key things we want to remember from this exercise are that our lines should always converge as a set not in pairs, never diverge from the vanishing point and due to perspective they won't be completely parallel.

So that's about it, in all other respect your work tends to come along pretty nicely, like the hatching lines and the lineweight.