Hi guys, I am needing some help to create a perfect cube using two vanishing points and I have got some issues on it, because I am not sure how to calculate the perfect symmetry of a cube in perspective.

I have seen to many cases like the next picture to draw a straight line (red line) to calculate the other segment projection.
Is this a rule of a cube in perspective?

I have found this attached picture in google
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and other question, is there some way to calculate the perfect blue long segment according the main black line with 4 cm?

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