On a concrete floor in the garage, thoroughly vacuum the floor all around you. Sit in a chair with your quilt on a table in front of you, vacuum to one side, and bag of down and a light weight scale on the other. Put a no-see-um filter between the vacuum tube and the tube attachment (call it the "wand"). Weigh the down with whatever clip you use to hold the bag shut. Put the wand in your lap, turn on the vacuum. Grab a handful of down and stuff. Grab the wand, clean your hands (hairy arms attract a LOT of down!) and clothes, and catch all the strays - air - table - floor. Repeat. Repeat one more time. Weigh the down. When you have the right amount left in the bag, close up the chamber in your quilt, catch all the stray clusters, then clean out your no-see-um filter (stuff it into the next chamber). Repeat with the next chamber.
Worked like a charm. Finished the stuffing and clean up in less than an hour (8 chambers after trying other things with the 1st 2) with only a few clusters left over at the end. After the first couple of chambers using this method I was able to guestimate how much down = 1.5 ounces and was able to do the hand washing (hand vacuuming) only a couple of times per chamber.
Thanks to everybody who offered ideas.