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Hi. I wouldn't use "noindex", so images are actually getting into Google's image search etc, but canonical sounds fine.

I don't know of any 'magic' ratio, but surely you should put nofollow on all user generated links? That's kinda what it was invented for. Normal links (or 'follow' if you will) should be used for linking out to sites you can personally recommend to your visitors - either the sites offer additional insights that your content cannot or doesn't cover, or it corroborates what you're saying (e.g. linking to a government resource or wikipedia). Does that make sense?