Hey Mitch, i know this is a decade too late, but the adjustable sliding knot you were looking for is called a Tautline. the lark's head / prussik is used as a friction knot, true but it has two lines coming out the center. its meant to grab in either direction on another rope ( climbing ) however the tautline is used to adjust the length of a guy line for tents. which is exactly what u were looking for. infinitely adjustable in trying different bridle lengths. cheers.