You are correct dragon. My first attempt I forgot to allow for my tarp to be on the insides of my knots. Luckily I cut my line plenty long enough as I knew I needed to experiment. For my setup with the WB Superfly tarp, plus around 4-3/4" for connection points on each side, the distance between knot centers ended up being 141-1/2" (11' 9-1/2") and my loop length 19-1/2". I also have right around 6" from the biner I'm using all the way down to the larks head of the continuous loop on my hammock.
I'm trying to work on DIY instructions on how to get everything at least close when starting to make one of these. It's really not too hard, you just have to take into consideration a fair amount of things like tarp length plus connection, how far you want the tarp above you, how long your loops are, how long you want your whoopies. I think I started with an amsteal length of 492" (41') and ended up with just over an adjustable length of 285" (23') down to 156" (13') Approximately.