John,
If you’re using a 3/4″ hem all around you would ideally only use up 3″ of material on the width of your 5′ x 8′ tarp.
Be careful with your hem stitching to keep your hem even and taut as you sew it. This is going to sound like rubbing your tummy and patting your head while balancing on a large rubber ball but here goes.
Mark your hem’s fold points and turn in the raw edge first and then roll over the two layers of material once again to the last mark. The edge of the two layers of material should line up with your last visible mark. Use pins if necessary to keep it all straight.
Start your hem by advancing the sewing machine by hand. Slowly make enough stitches to allow yourself to grasp the material behind the presser foot. Now comes the tricky part. Hold the material taut on both sides of the presser foot while guiding the material under the sewing machine’s presser foot and letting the feed dogs of the machine advance the material.
You will probably wind up sewing about six to eight inches of material at a time in short bursts. Doing it this way will help you avoid gathers and bunching of the hem.
As far as the pitches go with a 5′ x 8′ I’m picturing an A frame or a lean to style of pitch. You could maybe go with a fairly open flying diamond pitch which would give you a ridgeline of 9′ 5″ and a very “open” entrance. The foot end would likely be very “tight” as far as space goes. Headroom would be limited by dimensions but could still be fairly “open” if you were to use your trekking poles in an A frame/open triangle mode to create an unrestricted opening at your entrance.
Silnylon has a good amount of stretch to it. You can probably get away with using six tieouts and still get a fairly tight pitch using the stretch to your advantage. A cat cut ridgeline really helps in getting a tight pitch but if you don’t sew in a ridgeline seam you can’t gain its advantages. On the other hand you do get the pitch options. Ahhh the tradeoffs and compromises of life. ;-)
Check out Jamie Shortt’s 5 x 8 LytW8 1P Flat Tarp on his website. He also shows what I call a 1/2 pyramid or monk style pitch for this size tarp. He seems to get a very taut and neat pitch out of his 5 x 8 tarp.
Party On,
Newton