I don't think anybody knows how google counts back links apart from google. Their system is recording all the backlinks that are indexed but who knows which one counts and which does not? I guess this is why everybody wants as many links as possible. High PR sites can give you back link, depending on their webmasters. You can try to contact them and if you have a good quality site, quality content, you might get them to give you a back link. I would go for sites with similar content, the ones which might benefit from linking to your site.
Google never open hi algorithms no one can say how google count back link. my personal experience i told you it's count only quality back link.
i don't know how the process of count backlink, but you can see link to your site in webmaster tools google