+1 on the Klemheist knot. I stared out with a pair of these and a small s-biner on my ridgeline. After I realized there's no real reason for me to ever separate the tarp from the ridgeline I just removed the biner and tied the line straight to the tarp. This saved just a little weight too.
You can probably get away with some pretty small line between the tarp and ridgeline. At least for me, the line really only keeps the tarp centered where I want it as the ridgeline runs under the tarp. I have some really small cord that came along with an old tarp from Oware from years ago. It has a very abrasive sheath so it bites well on the zing-it.