AH, the lens alignment with the screen is almost always adjustable for table or ceiling mounting, so should not require the kind of lining up you mentioned. The concern is that the sides of the projected picture are parallel. This requires being able to adjust for what is known as "keystoning".

A keystone is shaped like a trapezoid, where the top and bottom are parallel, but the sides are not. A picture projected from above or below the screen centerline will exhibit keystoning, unless this is adjustable within the projector.

Larry
www.fineelectricco.com