FWIW, I just bought one of the same shells and decided to go with down from Wilderness Logics - $6.50/ounce of 850 fill. A bit more pricey that what you're looking at, for sure (I got 9 ounces for the partial length shell), but it'll keep it lighter, more packable and MUCH easier to fill than cutting up something else. It turned out to be 1 bag/baffle.
I hung the quilt off of an ironing board with small clamps (one at each end). I then cut open the bag, pushed it about 1/4 of the way into the baffle (it fit perfectly - it slid in easily but didn't leave any gaps for feathers to fly out) and alternated between shaking the bag with the down out into the baffle and shaking the quilt to move the feathers down to the bottom of the baffle. The first few took a bit of practice, but after that, smooth sailing. Kept everything much cleaner than what I'm sure occurs when cutting open an existing product, emptying it out, measuring it on a scale and then placing it in. My sanity was worth the extra $25.