...and what he wrote about? His question was whether you want to drop the projector by the same amount as the screen.

There are also a variety of projection patterns. I'm describing this from the point of view of an upside-down ceiling-mount projector: before implementing image shift, some projectors have the lens at the height of the center of the image, some have the lens at the top of the image, and some have the lens above the top edge of the image. These different types of projector would of course result in different required drop distances and the latter type would require the least drop of the projector.