It's easier to show than to tell how much chain slack is right. You should be able to touch the chain where it's halfway between the ring and the cog, and it should have I'd say about an inch of vertical play.
Another way to test to see if your slack is too much is to push on the chain where it meets your chainring. Try to push it up and over a tooth - ie, try to derail it. If you can't, then, it ain't coming off. If you think you maaaaaybe can, then, well, it's probably not coming off.