and then where you plan to sail (which you told us).

First, I suggest you not buy chain anywhere but a chain manufacturer and insist on calibrated chain with a calibration and WL certificate. Google them. They may even take your current gypsy and make the chain to fit for you. I got this done once, but not in the US.

Uncalibrated chain does not guarantee equality in the links, which you want for the windless.

You are right that the metric equivalents of imperial sizes such as 5/16 and 3/8 are slightly different. You might try buying 20 feet of what you hope will work and try it out.

All chain or combination. I think I would be really uncomfortable with anything other than all chain cruising. We don't even use an electric windless. In 20 to 30 feet of water you toss out 100 or so feet of chain and once you are sure you are set, you sleep very well.

There are rocks, coral, old boats and all kinds of stuff down there to eat your rope.

Up to about 40 feet with common windage I would probably stick with 5/16th calibrated, tested chain. Think about having an oversize link welded on each end to take a bigger shackle.

Our take on it. Results may vary.