A simple stop knot. I first used it to stop sliding floats.
take about 4” of the cotton (or thin nylon or thin pole elastic) hold it against the mainline, make a loop with the cotton and then pass the end 4 times thru the loop. Pull gently to tighten, then position where you want it and properly tighten.
cut loose ends to suit and add a dab of nail varnish as a fixed point - for reference.