It is all about the relationship between the image on the screen and the projector. Where is the image vertically and where does the projector need to be relative to that? Can the projector support the desired image size with a 20' throw distance from projector to screen? The answers to these questions are going to be dependent upon the image size and location and the projector. You can fix all the dimensions and locations and then try to find a projector that works, which may require a projector with image shift and/or an optional
lens. Or you can pick a projector and image size and location and adjust the projector location to fit.
For example, whether you have a 6'x8' image starting at 4' above the floor like a typical classroom might or a 16'x9' image starting at 10' above the floor to account for a raised stage and someone standing in front of the screen could make a huge difference in the resulting projector location and requirements.
A lift is nice for such applications and many models will support storage, show and service positions. However, lifts are typically not inexpensive and are more difficult to install.