Great, that's the website I was looking for.
According to this calculator to get a screen size of 120 inches with this projector you will need to place it 2.72m to 3.28m (8' 11" to 10' 9") away from the screen.
Thanks that puts it behind my wall! Oh well
its personal opinion. I searched and found that the recommend viewing angle for most people is 40degrees, for THX certification it has to be at least 36degrees. at 120", thats 2.6m wide, which to be 40 degrees viewing angle means you should sit about 3.6m back At this distance you should mount your projector 3m back from the screen. I have set my screen up with 50deg viewing angle and it seems fine for me.
How far should it be from the screen for, say, 120 inch image?