First: good luck with 250 boxes challenge. It also took me more time that I expected. The worst thing you can do is trying to make the 250 boxes on a row under 2 days... The best way to tackle the challenge is to draw between 25 and 30 each day (or 2 or 3 days each a week) and putting concentration each time you're making a box.

You said that you miscalcul a line. It's not necessarly a bad thing, it means that you try to put a marks for a construction line.

You can some line weight to make a constrast between the body of the box and other part of the line.

If you feel your first line is incorrect the best thing to do is not to use it. To make your submission sheets clean, it may be even better to use another paper sheet and keep the one with the line (and or shitty boxes) for another use.

Just like when you write a kanji, the order of the strokes is important. I strongly recommed you to draw your box by always putting the marks on paper in the exact same order. When you "draw a box" in 3d/ with three vanishing points, you need to start with a Y shape. The upper part of the Y will be the top of your box and are linked to vanishing point 1 & 2.

Be sure that the left part and the right are not perfectly equal else you'll have only three line instead of one. After you drew your Y shape complete to get a flat square (the futur top of your box).

Finally choose a 3rd vanishing point. It will be linked to the bottom of the former Y shape. This VP need to be rather far away, else you box will be distored but not too far else it will pass on 2D mode.

Good luck with the challenge.