Just finished making a rectangular 12 x 10 tarp using RSBTR's SilPoly material in olive drab. This is a very nice material to work with. I started with 8 yds and a measured 60 inches wide. Kyle cuts his material very close to what length you want - no extra, so I simply cut it in half and sewed it up as is. I didn't have to square anything - just started making roll hems and sewing the ridge up. I used a grosgrain ridge - (3/4 inch wide grosgrain).
I usually use 3/4 inch beastee d rings, but used 1/2" ones instead. There are 4 tie outs per side and I haven't decided to put pull outs on it yet.
Finished tarp length came to 142" long and 58" wide - used up 2 inches from hemming, but I'm still calling it a 12 ft instead of an 11 ft 10 inches tarp.
What surprised me was the final weight - 18 oz on the nose. It would have been closer to 16 oz if I didn't use 3/4" steel rings on the ridge ends. I also made 3/4 inch and 1/2 inch tie outs stacked together which added another oz or so.
Hung it to see what it looked like, but haven't really tested it yet. Over all I like this material - it's east to work with doesn't stretch when you pull on it. I didn't use all the tie outs - I'm a bit rushed today. Oh - also it has no cat cuts at all. When I start teaching again, I'll have to use it on the ground - I know, ACK!