I would suggest taking a look at this website. It is set up for anchors but the same formulas apply for moorings, the major difference being the scope used. Keep in mind that you need to take in the specifics of your situation. Any wave action will greatly increase shock loading. Additionally, how well your boat sits to an anchor (how much it veers back and forth in high wind) plays a big role as well. You can never have too large of an anchor and it is always nice to have a generous safety factor.