I just finished doing this over the weekend with a 110" greyhawk. You don't mention which mount you are using. I used the Sony mount, which I have regretted purchasing, even though things worked out in the end.
I first ceiling mounted the projector, leveled it as best I could. (This is where I hated the Sony mount, because it leaves a lot to be desired in terms of yaw and pitch adjustments). I displayed the test pattern on the wall, which looked good and thought about mounting my screen based on the pattern, but decided to use the recommended mounting instead. I'm glad I did.
1. Measure the distance from the top of the ceiling to the center of the lens and from one wall to the center of the lens.
2. Transfer these to the wall where the screen will be mounted. This will be the top center point for the viewable portion of the screen.
3. Assemble the screen frame (w/o material) and find the midpoint of the top portion of the frame.
4. Take the frame and hold it against the wall line up the midpoint of the frame with the wall markings you did in step 2. The wall marked midpoint should be viewable just under the frame. Check for level.
5. The frame has 3 vertical frame supports (at least for my 110") that are predrilled. While still holding the leveled frame against the wall, mark the positions of the predrilled holes on the wall. (I used a small drill bit - gave me a chance to check for studs).
6. Take frame down and remove the vertical supports from the frame.
7. Mount the vertical supports on the wall, checking for plumb.
8. Snap the screen material onto the frame following Stewart's instructions.
9. Mount the frame onto the supports.
10. Turn projector on, display test pattern and adjust projector to fit screen viewable area.
I did this and had no keystoning problems at all. My only problem was getting the image level - which was the mounts problem. It had more play than I would have liked and no good way of adjusting tilt. Once I got the tilt problem worked out, the image was right on target.
A couple of notes: First, you need at least two other people to help you. Use a long level, it will make it easier. I also didn't hit any studs, but the toggle bolts that Stewart provides worked perfectly.
BTW, I played with the idea of using the test pattern to determine mounting position. I'm glad I didn't do it that way, since it turned out that the screen needed to be lower than what the test pattern was showing, even though I didn't notice any keystoning.
Hope this helps!