The best reflector for this is, believe it or not, the 7 inch that came with your lights, with a barn door set (trust me, it's necessary). The 7 inch will completely cover an 8 foot silk from 6.7 feet away (I usually pace off 6 feet and say "close enough"). That produces a round spot of light 11.2 ft in diameter, which is why you need the barn doors, to eliminate the stuff that spills past the silk at top, bottom, and sides.
Paul's set costs about $110 for the Mod Lite frame and the barn doors, and it's great, the frame also lets you mount snoots. The barn doors are rectangular flaps, and I prefer expanding barn doors, so 2 of my Paul Buff 7 inch reflectors are outfitted with Lowel Omnilite barndoors. You "commit" a Paul Buff reflector to Lowel barndoors, it took two people stretching the barndoor frame to get it on the reflector, and I don't think I can ever get it off again.
At 6.7 feet, you have 40 degrees divergence from perpendicular at the corners of the scrim, which is enough to give a substantial hot spot in the center for most silks. If you've got enough light that you can afford to lose a stop, backing up to 9.4 (or 9) feet will decrease divergence to 30 degrees, and the light looks much more even. And makes the barndoors ever more necessary.
If you can get the space, the 8.5 high output is a great reflector behind a flat. The calculations say it has to be 13.6 feet back to cover the corners of an 8 foot flat, but I usually just pace off 12 feet. This combination has as high a light output as the 7 inch reflector at 6 feet, but it's so much more even. One of my 8.5 inch HORs wears a Norman 10 inch barndoor set with a homebrew wire mount. (Hey Buff guys, how about barn doors for the 8.5 HOR, now that you've got grids?)
The PLM theoretically can light an 8 foot flat from 13 feet, but I don't want to see how you flag the spill. ;)
I also sometimes light a flat with a 16 inch Photogenic, modified to fit the Paul Buff mount, with Photogenic's 2 door barn door set. I should have the 4 door. Say $300 for reflector and barn doors, and about 20 minutes with a nibbler to fit it to the Buff mount.
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