In the HEAVEY equipment industry the largest concern for a chain is the link sheer strength when used with any kind of holding device, just like the pin in the shackle. a good forged hook doesn't add any side load that would change the sheer strength of the chain link. When I worked in an industrial welding and fabrication shop we had 3/8 inch hi tensile chains that we used to lift drilling rigs that weighed well over 100 tons, four chains, with forged hooks. That's how we painted the underside of these units. These chains had to be inspected before each use, and magnaflux tested every week. If the foreman saw you with a welder or torch within 2 feet of one you could pick up your check that day. They were very serious about the chains for overhead lifting, even more so about the hooks, if we could we wrapped the chain around a round object, if not we had load spreaders that attached to the chain. Even a chain that is used and abused on a regular basis will give incredibly long service if not overstressed. My boat is not large by some standards, probably average to maybe a little on the heavey side, but I'm pretty sure a 3/8 inch chain and a good hook can handle way more than it can. The hook and chain won't be the failure point.