Is the screen to tie the plants back to, or to go between the plants and bulb, protecting them from the bulb?
If the screen is to block the plants from touching the bulb, you'll probably want a diameter of 2-3ft, so you can get as close as 12-18", if you have plants narrow enough to fit that tight, that close to the bulb.
If you're going for 4-8 decent sized bushes, and want to tie the plants back to the screen, you may want a max diameter of about 8ft, to keep plants within 3-4ft of the bulb. If this is the case, your 9x9 may prove a little tight to work around in behind plants.
It depends on numbers / size of plants to a point. I believe a 1k will effectively penetrate about 4ft in general, so I personally wouldn't want the back sides of my plants farther away than that, but I wouldn't want them closer than 12 or 18" to the bulb either, to minimize heat stress.
Hopefully this helps some, but I'm sure there are folks around who can help with more info too... Good luck!