The reconstruction of a stereoscopic pair of images from a single "flattie" requires some thought. You cannot go back in time and make a second image.
If we assume that we have a right-eye image, and want to create a left-eye picture, we have to bear in mind that if that left-eye view had been created in past, it would have "sneaked" past object being photographed or painted and have captured slivers of background invisible from right.
So a human figure in front of a pastoral scene would have pieces of countryside fringing its left.
As wheatstone - INVENTOR of stereoscopic imaging - pointed out in his classic 1838 lecture (http://wehner.org/3d/first/index.htm ), perspective difference between eyes is NEGLIGIBLE for distant scenes.
So we would keep scene intact for right eye, and over-paint human figure with some imaginary background until pastoral scene has been reinvented - at least along left fringe of figure.
We would crop left-hand edge of left eye image and right-hand edge of right-eye image, so that when superimposed upon each other, such as by projection with polarized light, left image lands upon screen about two-and-a-quarter inches (62.8 to 64.8 millimetres) to left of right.
The eyes, viewing perhaps through polarized spectacles, will now be obliged to view in parallel if STEREOPSIS (the lack of double-vision) is to be attained. This tells brain that background is at infinity.
But what of human figure?
Perhaps we want to position that figure at six feet distance - about two metres. This would suggest that reconstructed right-eye view must be offset eighty, eighty-one or eighty-two printer's POINTS to right of where it originally was. We can see at once that specification for landscape reconstruction is a sliver of image to figure's left that is eighty, eighty-one or eighty-two points wide.
And specification for figure itself will be that each point of image-shifting will represent about an inch of backward/forward movement - about two-and-a-half centimetres.