Right. Use smaller diameter cord and make it into a loop using a fisherman knot. Make a prusik around the main line you want to bring the slack in on and hook the other end of the small cord to the tree strap (I don't know what these look like but I imagine worst case you'd need to buy a 'biner for it). With no load on the line pull the main line through the prusik until it's where you want. When you load the line the small cord with the prusik will take the load instead of the main line...fully adjustable!