What do we have to work with and to fill for bottom insulation?

Clark pleats the pockets from front to back. They span 12" (30cm), 12" (30cm), and 15" (38cm) of the hammock bed, from head end to foot end, but the gathered fabric measures 19" (42cm), 19" (42cm), and 22 1/2" (57cm), the difference taken up with the pleats, elasticized at the top.

Half the difference between the span and the fabric length is the implied permissable height of an uncompressed insulating pillow. (19-12)/2) = 3.5" (87mm). That is the amount of loft of 0F / -17C down-filled quilt.

The pockets are paired, left and right, each 25" long from top edge to bottom centerline, but without additional radial fabric. So a rigid 3" thick batt 19" long would take up some of the pocket fabric toward the bottom of the hammock bed and some at the top of edge. In practice, there would be some tapering at the top due to the elastic Clark has sewn into the top gathered edge for closure by Velcro.The pockets may then be estimated to have an effective depth, for area an volume calculations, of 21" (525mm) each and twice that, 42" (105cm) for a pair.

How much insulation, then for a Clark NX-250?

The pockets span about 40" x 42" ~1700 sq in.
Instead of a large triangle going from the last pair of pockets all the way to the gathered end, I would add instead a smaller rectangle.

An additional 10" wide (x 42") quilt or pad would result in length-wise coverage about equal to that of a 2/3 fractional quilt, covering about 2100 sq, a little more than 1 1/2 square yards.