Happened to me on the weekend. Scary eh. especially when you come up and there's nothing but sail? Like being under ice

You just have to remember to not panic, reach down to where the hook is and release. Only scary the first few times, you just have to expect it from time to time, less and less.
There are harnesses with quick release buttons, although I'm not sure it wouldn't be just as quick to release the line from the hook. I've noticed that the toggle on my adjustable lines fits perfectly between the bars of the hook so I wrap tape around the hook to prevent the toggle getting very trapped.
- Line length about elbow to bottom/middle of palm.
- about 3 fingers apart
- about 1/3 between mast and clew of sail
See here:
www.guycribb.com/userfiles/documents/The%20Truth%20About%20Harness%20Lines.pdf