Length: floor to chin is NOT enough. For a start, when you are asleep your feet tend to point, adding at least 8". Second, there is nothing worse than struggling with the top of the quilt trying to pull it up another few inches around your neck on a cold night. Thirdly, when the quilt is stuffed the bulges from the down shorten the length anyhow. Be generous with the length: you won't regret it. Shaving inches off here can be very false economy if it leaves you cold. I would strongly recommend AT LEAST 76".
Now, buying fabric. I don't know where you are buying it from, but I am sure that they will sell in smaller increments than 1 yd. They may not sell *less than* 1 yd, but above that they are always more flexible. Surely! Make sure you aren't 2" short! False economy.
Sewing the long edges together: all I did was run the baffles to within about 1/4" from the edge, then I sewed the long edges together. The way the baffle gets compressed next to the seam will stop down migration. You can get more fancy and try to include the end of the baffle in the seam, but it really is not necessary imho.
Baffle height: for 9 oz of 800 loft down I would suggest a baffle height of only 1", not 2". Yes, this means the loft would seem to be restricted at the baffles, but that is not how it works in practice in my experience. It bulges more in the middle of the chambers. On the other hand, if you make the baffles too big the down is too free to move around, and it will tend to migrate around away from the centre of the quilt to the edges. Needless to say, that is undesirable on a cold night.
Note added: this is why 'overfilling' seems to work so well: the extra down helps to prevent down movement within the chambers.
Calculating down loft volume is a marketing gimmick. The way the shell bulges defies all calculations!