I've never made a down garment, but I can say in general you need to calculate the volume of the filled baffles and divide it by the fill power of the down. So you know the thickness of the baffles (1.5") multiple that by the surface area of the garment (in in^2) to get the cubic inch volume of the filled volumes. Take that volume and divide it by the fill power (850 in^3/ounce) and that gives you the ounces you will need to just fill the baffle. Now, fill power is usually a marketing lie so you need to over fill the baffle to get it to remain nicely filled while in use. I believe I've heard 30% as a good overfill (so multiple the calculated number by 1.3).