Hard to formulate the question, so here's what I'm not asking (unless it answers it and I am missing it...)
I do not mean mirroring a rectangle in perspective by finding the center point with crosses, and then drawing a line from one corner through the midpoint of the rectangle to extend it perfectly in scale along the axis.
I do not mean taking a distance, or height, and moving it around in 3d space to another point (like receding telephone poles)
I mean transferring the width of something in perspective, to basically make a perfect square from it, without an ellipse. I know that ellipses will form a perfect square if they tangent all lines of the square in the middle.
But due to ...some reason the software I am using, has a really crappy ellipse tool (Opentoonz) and I don't have an alternative right now...so, if there is another way, I'd be happy to hear it.
Note:I want to know this for general drawing skills, I'm just mentioning the software cause this is where not knowing it is biting me...it's for a personal thing. Trying to learn how to animate flaps and stuff.
Even a nudge in the right direction would be appreciated, or just some link to a video or text, no matter how low effort, anything than my own, at this point, paralyzed mind.