Silverhawk - I don't even try to fray the ends; simply heat both with a match until I get the size knob I want. As for length, I tie the loop as small (e.g. short) as my hands will allow -sometimes even use needle nose pliers to coax the string through the knot. When I have both knots formed and the ends of the string melted, I insert the needle nose pliers through the loop and pry them apart as hard and as far as I can (I do this several times to stretch the string as much as possible). This will pull the knots tight and creat the length needed in the loop. If the resulting loop is longer than I wanted, I'll untie one knot, retie it even shorter, cut the excess string, remelt, and then restretch with the pliers until I get the length I'm after.