800 cubic inches is a measure of the volume the 1 ounce of down occupies. Therefore, if you had a baffle compartment had an area of 20 inches by 20 inches (400 inches sqaure), an ounce of 800 fp down would give that area a loft of 2 inches (400 in^2* 2 in= 800 in^3). If you have a quilt, like the JRB No Sniveller that is 48"x78" and want 2" of loft, then you find the volume that you will be filling and divide that by the fill power of the down (ci/oz) to find the amount of down (in ounces) that would be needed. So in the case of the NS, 78*48*2=7488 ci. 7488/800= 9.36 oz of down. They use 10 oz of down to really puff it up and give it a bit of an overstuff.
I made a down quilt last weekend actually and wanted it to be good down to about 30-35*. After looking at all the specs of a variety of quilts and lightweight bags I decided I would be fine with 2" of loft. My quilt had a volume of 7068 ci, so I would need 8.8 oz of down. I ordered 9 oz to account for loss while I was stuffing.
I hope this helped clear things up a little.