I used to have prussiks, but they would always loosen on the slippery lash-it, so I went to this system.
https://www.hammockforums.net/forum/...ght=Continuous
I love it and it sounds like you have the necessary Dutch bling for it. Instead of using a key ring (which I mangled on my first use) I attached the tarp fly to my tarp with a continuous loop of lash-it (I spliced it in place so it can't be lost). I use a 30' ridgeline with about 5" on my Dutch hook and tarp fly.
Back to your question. I don't think I would go any shorter than 28', but I think it really depends on how large a tree you are connecting to. Here is a quick calculation. 2 trees 18' apart, each with a diameter of 2', tarp with 12' ridgeline.
The ridgeline will go the length of the tarp (12) + to tree1 (3) + around the back of tree1 (3.14 : circumfrence is pi * diameter, but we only have to go around half of the circumfrence so it is just pi feet) + back to tarp from tree1 (3) + to tree2 (3) + around the back of tree2 (3.14) + back to tarp from tree2 (3) = 30.28 feet