That is a good point - you need to find the knot you like and can do in the dark when its raining with cold hands
With that in mind I don't see any reason you can't just treat it the same way as you do your adjustable guy lines only reversed, so that the fixed knot is at the tree and the adjustable near the tarp.
Personally I run a reasonably taught line first, from which the tarp is hung, then use 2 miniclips and prussics at each end as this give the opportunity to reposition the tarp along the ridge with little fuss. It also make setup and packdown easy, as I keep the loops attached to the ridge, set up becomes little more than tying to 2 trees, sliding the prussics to roughly where they need to be, clip on to the tarp loops and then tension.
