OK, so, it's done.
Centered zip, draft collar/extension, no draft tube. Functional finished length is really about 56 inches, although I cut fabric at 62 inches. I added the draft collar bit to try to bump up length to 58 inches. Works out all right for now. The zipper problem I was having was that I wasn't paying attention to the orientation of the double-pull (ie, pull tab on inside and outside of slide), and was trying to put the slide on the zipper upside down.
This is a hyper-super-mega-efficient cut. I mean, I think my Summerlite is a pretty roomy bag. This one is technically really too snug, given that it's tight enough in my (narrow-ish) hips to compress the insulation a little. Outside chest dimensions (all dimensions measured flat) 26 inches, hip 20 inches, foot 8.5 inches. My original plan was 59 inch chest girth, cut out of 60" fabric. But the fabric started out at 58.5 inches, then I lost some functional girth after stuffing with down. Original plan was 32 inch footbox; the original panel I cut was too large, then too small. The third attempt seems to be just right.
I think because of the hyper-efficient cut, this bag is HOT! I only used about 6 ounces of down (yeah, yeah, I know... that vest I did used 12!) but it lofts beautifully and I did calculate mass of down required for each baffle. (In case you're wondering, 5 inch baffle x 1.9 inch desired height x width, ie 32 inch. Take that number and divide by fill power of down.) I start getting warm pretty much as soon as I slip into the bag.
My original working calculations varied from an expected finished weight of 12.6 to 13.2 ounces. The finished weight is 14 ounces (yes, exactly 14, unless you want to get to thousandths of ounces). Shell fabric was about .08 to .09 osy high from spec, which was about .8 ounces. I also hadn't planned on adding the draft collar, which added .75 ounces. I did use a little less down than originally planned. I messed around trying to use it as a quilt yesterday because I could have kept the weight at about 12.5 ounces. But the footbox is so tight the bag automatically rolls over with you. Zipper was needed.
Thoughts: I off-centered the draft collar so there wouldn't be a gap right over my neck. I should have put the drawcord on edge closest to my head, instead of where the main bag ended. Next project will allow more girth and length (say an extra 3 inches added into planning figures for cut fabric?). I used baffles cut at 1.5 inches and sewn to a functional height of 1 inch, allowing an "extra" half-inch loft on each side; I believe in having the greatest density of down possible. I cut the footbox baffles a half-inch higher and stuffed accordingly; my feet get cold easily at night.
Bottom line, the bag is 2 ounces over goal, but only about .8 ounces over calculations. More importantly, my overfilled SummerLite weighs 20 ounces, this one weighs 14--I just cut 6 ounces from my pack, for a bag that's at least as warm!