The spreader bar knot is a non-slip 'easy to undo' knot, which can always be retied.
To adjust - shorten the rope at one end only. Loosen the knot and pull the rope through until the correct length is obtained, then retie the knot. When retying the knot, it is important to always leave a 4cm rope tail for safety - cut off the surplus rope then burn and seal the rope end.
Finally, readjust the snap link, so the spreader bar is always suspended horizontally.