The positioning depends greatly upon the
lens and the relationship to the audience, as well as
practical considerations such as how you can mount the
projector.
Most very short
throw lenses (<1.0:1) rarely have any
keystone correction capability and defeat any
lens shift functionality, thus almost always requiring that the
projector be on the centerline of the
image with those lenses. Most other lenses will have the
projector ideally located near the top or bottom of the
image.
If your audience area is sloped up away from the
screen then consider putting the
projector towards the bottom of the
screen. If the audience area is
flat or below the
screen, then having the
projector towards the top of the
image may work better. However, the
projector location is often dictated more by
practical considerations than it is by technical ideals.
Somewhere in the
projector's on-screen menus is probably a
projector configuration option. Typical selections would be front projection tabletop, front projection ceiling, rear projection tabletop and rear projection ceiling. If you can't find this and don't have the
projector manual, you can usually find manuals somewhere on the manufacturer's web site, typically either with the product data or under technical support.