I saw where I guy you some stuff kinda like duct tape where the tape hangs out a few inches. He put a plastic washer in the middle and poked a hole for his guylines. The other way with out a washer or grommet the tape would tear
Could have been me. I've documented most of my experimenting at BPL. Anyway, if you make a loop of tape, it will hold pretty well. Otherwise, using a plastic grommet (preferably with a flat side toward the outside) is the ideal it seems. Don't poke a hole through the tape unless using a grommet or something to prevent the line from tearing through the end.
For a ground cloth, I use the ‘indoor’ polycro. It’s easily ½ the weight of the ‘outdoor’ polycro. And they last me easily 100 nights. But for a tarp, the ‘outdoor’ version looks tough; though I’ve never tried it tarping. A tip of the trade: let it sit out in the sun before you cut it to shape. You don’t want a perfectly cut ground cloth shrink on you while backpacking (because of sun or hot backpack).
Indoor and Outdoor are usually the same thickness 0.7-75 mil though the density may be different. There are a few kits specifically labeled "heavy duty" that are 1.5 mil.
It won't shrink if set up in the sun or in a backpack. It may deform if you leave it compacted in the sun where the little air pockets hold much more heat! My son sort of ruined my tarp that way so he could mow the lawn. I still used it for 6 nights in the Wind Rivers since I didn't have time to make a new one.
I normally have gotten my kits from Walmart (on clearance or online). There was one kit (wish I could recall which one) I'd gotten somewhere that was useless for even a groundcloth.