There are a number of threads recently about how if you 30% over-stuff on horizontal tubes, the down won't all slide to the edges leaving an empty place on top
I haven't made a down sleeping bag, but I'm thinking about it, so consider this what you wish : )
0 degree F - maybe 2.5 inches of loft? If you wore some stuff inside?
Maybe 4 square yards for a sleeping bag?
If you used 0.7 oz/square yard shell and liner - 5.6 ounces total
4 square yards = 5000 square inches * 2.5 inch loft = 12500 cubic inches add 30% = 17000 cubic inches. If you use 900 fill power down that would be 19 ounces.
Plus, add fabric for baffles and zipper - 2 ounces?.
Another thing about down not shifting in baffle, is to have the baffle width small enough. I think if you had 2.5 inch loft, you would want 5 inch baffles, but that's just a guess. Maybe look in a store, find sleeping bags