How can I find a proportional length on a perpendicular line from a perspective?

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I am working on a project where I am mapping images of buildings onto 3D meshes and I want the dimensions of the mesh to be proportional to the building in the image.

I have six points for a given image that describe two sets of vanishing lines and a vertical line where they intersect. I want to know the length of one side of the building proportional to the other. I can assume that the lines intersect at a right angle and that the vertical lines are the same height in real space.

Referring to the linked image, is there a way I can find the length of a given an assumed length for b?

enter image description here