Back to backlinks - I'd say that off-topic backlinks are not a problem. Google can and does ignore such links. And it's not really a percentage thing, either. What any site needs to thrive is links that come from independent sources - especially links in the content area. In other words, evidence that other webmasters value your content and recommend it to their visitors.
For this reason, a lot of mechanical link building efforts don't give a lot of benefit. Google isn't looking to give high value to links that a webmaster can place on their own.