Tilting the screen downwards should give you excellent results if you can deal with the tilt and feel it is not noticeable. This allows you to leave the projector in a more proper 'aligned 'position with respect to the screen. If you are going to utilize a high gain 'reflective' screen with a floor mounted projector, then tilting is the only option available to you if you wish to keep the gain offered by the high gain screen, without re-applying the projectors position.
You will gain overall better focus and alignment out into the corners of the image with this set-up, as opposed to not tilting the screen where you would end up with focus problems top and bottom, bad alignment and focus in the corners, and a dim, if not noticeably (brightness, possibly color as well) skewed image.
High gain materials are very sensitive to position and alignment to viewing area, almost to a multiplier that is linear with respect to their gain ratings.
The reality of the situation is that they actually are exactly problematic to a level that is exactly linear and in line with their respective actual gain ratings. HOWEVER, the human eye/brain combination is not linear in it's response to different 'so-called' problems and everyone's level of perception in those areas may be largely the same, but their personal interpretations, biases, and tolerances come into play, making multiple screen types, variations, executions, etc., and that little insanity called an 'opinion' come into play.
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