I was getting frusterated with that too, I'm at the same point in the challenge, but it seems that the back corner of the boxes ends up almost always being off because of tiny errors in the rest of the drawing compounding. Basicly you should try not worry about it too much, cause it will probally be off most of the time, and just do your best it should get better as you move along in the challenge or at least thats my understanding of it.

Once you get to the back corner, it's location is already determined by every other mark you've put down. If you have drawn everything else we'll, then your back corner will have no problem and your lines won't diverge. If you have drawn your other corners and lines with deviations, then your back corner won't work and your lines will diverge no matter what you try to do to fix it. Therefore it's a distraction in the sense that there is nothing you can do to make your back corner better by the time you get there. If you keep getting divergence, then you need to work on the other lines and corners first.