I just use a small peice of 550 to tie a tuatline hitch to my CRL. Easily adjustable.
I don't want my tarp 'permanently' attached to the ridgeline - knots in Zing-It don't untie for me, and I can't be bothered messing with button knots aka soft shackles.
Lots of ways of doing this stuff.
They're the doohickeys in the bottom left of this image:
However, you can do what Fallkniven suggests if you are a non-splicer. Just take a length of cord and wrap it three times around the ridgeline forming a prusik, like so:
You'll have two ends of the cord dangling (No knot tied yet to complete the loop) and all you have to do is tie it to your tarp ridgeline tie-out using an overhand knot (or whatever tickles your fancy). Voila! You have a prusik that allows you to slide the tarp on the continuous ridgeline and center it.