I didn't get too scientific about it as I am sure movement in the hammock, pull of the cords, etc. would throw off my calculations anyway.
So, I added twice the loft of my insulation to the outer shell with darts at the corners and along the side. The thinking is that the outer shell will start away from the inner shell at the edges and the distance will remain pretty constant across the UQ. In my case this was an additional 7" (3.5" loft) like Dutch said.