General:

You keep trying to correct your lines by drawing one on top of the other, which is:

a) Pointless because it's an exercise and beauty is far from from being the goal, not to mention the fact that the extensions can (and, in my opnion, should) be based on the planning points.

b) Harmful because it hinders your confidence, making your lines uglier on the long run.

You use one extension color per box, but making each axis (X, Y,Z)a different color will make it easier to find mistakes.

First 50:

You understood what you were supposed to do. Some lines extensions wobble, maybe you free handed those? Your main problem seems to be the inner edges being more off than than the rest, which is very common, since those are where all your little innnacuries are piled on.

Next 50:

Most boxes have a high convergence rate, which is shown by the extensions converging either in the page or very close to its borders. Nothing serious, but it can certainly undermine the effectivity of the challenge.

Last 150:

Sometimes the back planes of the boxes are way off, sometimes this has to do with you not finding a good

balance between the three hindmost edges despite having enough leeway for that, but a lot of the time it's because one of the three front edges of a set doesn't converge as much as the other two.

Also, sometimes the middle lines are weird, this might help you understand why. Basically, the closer they are on the page, the more paralel they are

Otherwise your boxes are good.