We had to get a new sewer last year - boy, was that ever expensive! But at least now, th....
Oops! Sorry! Wrong type of sewer! <>
Okay, seriously Jeff, Is this your first project sewing nylon material? A down quilt is quite a project, and if you're not familiar nylon, you might want to start off with a more limited project - how about a pack cover? Or a pair of gaiters?
Are you doing a sewn-thru or baffles? You might want to check some of the outdoor fabric places for patterns - these would make your job a lot easier!
I would recommend a 5/8" seam, then add whatever foldovers, etc, you're going to need. A shockcord tunnel could be anywhere between 5/8" to 1" (times two if it's folded over). Remember to include fold-over seams at the ends of the channel!