I'm afraid your question is too broadly worded. I don't mean to put you off, but projectors differ greatly in design, and it's impossible to generalize about where a projector will be situated in relation to the screen without knowing something about the specific projector, your room dimensions, floor/ceiling mount, screen size, etc. etc. I would suggest the following approach.
1. Select one or more specific projectors that seem to meet your needs and budget.
2. Pick a target screen size which is appropriate to the dimensions of your room and the projector you are leaning towards.
3. Check out the manufacturer's web site for info on throw distance and projector positioning relative to the screen. Sony, for instance, has an excellent program called PJCalc which will give you exact placement info, right down to the inch, based on screen size and projector model.
4. In doing this research you will doubtless eliminate some projectors as being unsuited to your needs, but will understand the issues sufficiently well to direct more specific questions to this forum. But ... first do a search on the topic that interests you. You will probably find that your question has been asked and answered many times before.
QUOTE]Originally posted by dinodog:
I have a question about the relaionship between projector & screen.When mounting the screen,what factors do i need to think about? For example, does the top of the screen have to be at the same height as the lens on an LCD projector? Can i angle the picture down or does the projector have to be mounted in a flat position. & what is KEYSTONE correction exactly?IS this a vertical or horizontal adjustment technique? Hope someone can help.