So, I thought I'd tweak the design a little bit.
You'll see I've shortened the tarp to 11' x 8', (5'6" wide at the edge.) This makes it a little wider too and gets my diagonal seam up to 9'8", which adds interesting options for hanging it.
The finished sides end up at 4'6" in width so prior to assembly I should be able to get what I need out of 62" wide sil. (give or take an inch...)
This design requires 18' of fabric or so, so 6 yds. Still better than 9 yds...
EDIT: Unless it doesn't work!