looks good. My first tarp was an 8x10 out of some super cheap uncoated material from walmart (I got over 15 yards for like $2 total- crazy)...so it was low risk, only time lost if I screwed up real bad. I ended up silicone coating it myself after it was done.
but- to my point, the flat felled seam isn't that hard with some practice. One tip I can give you- for a 1/2" seam, draw a line 1/2" in from the edge. Pencils write on silnylon if your go back and forth a few times over the same place- or if you don't care, use a sharpie. Dunno if a washable marker would work and come off easy when you are done or not. But that gives you something to line the fabric up to while you are sewing. My last project I didn't even need to pin it. It wasn't to hard to fold the fabric over as I was sewing. It takes some dexterity (not that I'm an expert) but use your fingers to try and hold the fabric taut in all directions as it's going through the feed dogs. My hands usually end up cramping doing long seems...I have no idea how those sweat shop seamstresses manage.
Anyway, my next tarps are going to be smaller ones like yours, then my ultimate goal is a big 11' pyramid... But that might still be a ways down the road.
ps. Do you think you could post some close up shots of your tieout reinforcements? Did you bond them or just sew them? How are they holding up under tension?