What I would do is as follows since I take it you dont have the projectors manual.............
1. Simply set up the PJ about where you think you might want it. Turn it on as project a rough image on the wall ( screen wall )
2. Take some measurements at this point. Is the projected image going to be big enough to fill the screen or is it too big? Is the bottom of the screen to close to the floor? If so how high do you have to raise the projector? build a pedistal for the projector if necessary.
3. Sit in your seating arrangements and see how it all looks. You may want to place someone in the front row and you sit in the back row and see if you can see the screen clearly. If needed you may want to build a riser to place the rear seating higher then the fron row to allow more visibility from the rear. ( not to mention that it would look cool )
4. If all is ok and in the realm of the projectors adjustments, you can mount the screen. Keep in mind though that you were projecting the image directly on the screen wall and the actual projection screen may sit off the wall, possibly 1" or so closer to the progector. You may need to refocus and re converge (if CRT) to get a good image.
These basic steps helped me to set up my SONY CRT before I got the manual. I am within the projectors throw distance for a 100" screen and I was able to get the unit fully converged for my 100" screen. I of course will be ceiling mounting the unit and therefor need to make subtle adjustments after installation to insure good picture quality.
If you use these as a guide line you should be able to answer some of your questions. REMEMBER this is not referance material so you will need to make changes if you are not seeing the results you are looking for. This will however help you to get the room constructed with basic guide lines to keep in mind.