The first rule of projector placement is that the projector must be a certain distance from any specific screen size. Two factors: Screen size, and the projector itself.
Distance from a specific screen size will be a range, and can be found using the excellent projection calculator tool here...
http://www.projectorcentral.com/Optoma-HD20-projection-calculator-pro.htm
The range is determined by the proector in relation to the screen, and every projector tends to be a bit different - sometimes significantly different.
The Optoma HD20, with a 100" screen as the link above references, can have the lens to screen distance anywhere from 10' 11" to 13' 1" - Only about a 1 foot range, so placement needs to be careful, and anywhere in that range is good.
With longer zoom lenses, like that on the Panasonic AE4000, I would shoot to have the projector about 25% back from the closest position. So, if the potential was to put it 10-20 feet from the screen, I would like the lens to be about 12' 6" from the screen. This avoids the extremes of the zoom range which can present lens aberations which should not exist, but sometimes do.
Likewise, I try to avoid the full extents of the lens shift range as this potentially introduces anomalies as well.