The loop knot can always be loosened and moved to suit any spacing or situation. If the lead is tied too long the horse may get a leg over the lead or may become tangled if it rolls. Again, the high line is to keep stock from damaging trees or their root systems. If the lead rope is allowed to slide along the high line, it defeats the purpose of this method unless stops in the high line approximately 10' away from each tree are utilized. Half inch hemp rope makes a good high line.
Nylon is too stretchy.
Multi-filament poly plus rope is best. It will stretch more than hemp, but is stronger, lighter, and will not soak up water. Many horsemen use the lash ropes from their pack saddles for the high line. A hitch rail can be easily constructed by tying a solid pole between two good-sized trees if a deadfall pole can be found. The horses should be tied on the opposite side of the tree from the pole so that the strain is against the trees rather than the rope-ties.