One of the problems you are going to find with a swinging mooring is that your boat will sometimes ride against the buoy. If you have lines which are too long then the buoy will scratch the hull (and removing any decorative stripes) further back on the hull. In my opinion, if you do not have a very vertical stem then you may be best having a length that just stops the buoy reaching the hull. But it still need some length so it can bob up and down without pulling on the hull too hard. Rather than getting a recommendation I would suggest you try varying the length until it looks right.