Here's a pretty good video on how to make one. I followed his process (but didn't get the strands twisted. LOL
I made mine a 36 inch sling and used two 26 foot strands for the outside parts and one 38 foot strand for the center. I only had about 2 feet of scrap per strand when finished so I think I have the math figured out for lengths. I factored 2 feet of scrap when I calculated my lengths and it's what I ended up with.