I also remember 3 screen heights as the optimum viewing distance, but 1.5 screen widths is just as applicable. However, these are really optimum distances, you can go closer without having an unwatchable picture. With the Sharp 9000, you could probably get as close as 1 screen width without visible screendoor, but a DVD obviously wouldn't look it's best. It's a bit of a compromise on quality, but *you* don't have to sit in the front row! :D
Also, Scott is right, your screen is quite large and the Sharp may have some trouble producing a bright enough image with the Greyhawk. You should definitely wait and see what the Firehawk beta program produces, the product is showing at CES and should go out the beta testers in January. It's designed specifically for projectors like the Sharp 9000. You should check out Don Stewart's post earlier in this forum, they did some measurements and made recommendations on screens for the Sharp 9000.