Most people seem to use 4-6' per corner for guylines. Err on the longer end of that range if you plan on using trekking poles to set up your tarp in porch mode.
That having been said, I would recommend
Lawson Reflective Cord over Zing it for guy lines. When you inevitably have to get up in the middle of the night for one reason or another, it makes it much less likely that you will trip. I would also recommend some sort of self tensioning system since your tarp is made out of silnylon. Silnylon stretches a fair bit, and stretches more over time, especially if it absorbs water. The stretch dissipates once it is no longer under tension, but if you don't use some shock cord as part of your guy line system, you will rig your tarp up nice and taut, go to bed, and wake up to a floppy tarp. My favorite method of dealing with this problem is
tarp worms.
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