The skin I just made for my large 12 X 12 neo tarp is just one long skin (inspired by whoever posted that the big air ball in the center can sometimes be a problem).
I just bunched up the tarp (while still hanging) and grabbed the fattest part and measured around it. I then sewed the skin to that size. Hardly scientific, and most likely not the lightest ones going, but enough for me to get started. Once I get some sil and learn to sew that, I'll make some good ones.
The harder I work, the luckier I get.