I've found that having the tarp ridge line of diameter and the prussik another helps the knots to grip better.

Of course, it might just be my imagination.

175lb braided trot-line works good for the prussiks. I found some other cordage in the marine dept at WallyWorld that is polypro and doesn't stretch and it comes in orange and lime green so that I can see it (you might prefer less obvious colors, but I don't do a lot of stealth camping.) It's not super light weight but it's not exceedingly heavy either. I've been using it for the ridge line and it works well.
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