You are correct, it is the total value of all your backlinks that determines your page’s PR. Not all backlinks have the same value and many are virtually worthless, therefore the number of backlinks cannot be used as a reliable indicator of backlink value. Even the PR alone of the page that links to you cannot tell you the value, you must divide that value by the total number of links contained on that page.
To complicate things further, to determine backlink value you must know the current PR of the backlink page which is updated frequently but reported on only once every few months.
An important thing to remember is that search engines don’t rank websites they rank web pages. Naturally, it follows that the PR of the linking page is what counts, the website itself doesn’t have PR.
It seems to me that PR, how it’s valued, and how it impacts rankings is a mystery to most Internet marketers and even many SEO’s don’t really understand it. The PR of your web page has very little direct influence on where you page ranks in the SERP. That is determined primarily on the relevance of your web page.
Where PR does play a major role is in how it influences the value of your backlinks. It appears that Google uses this as a weighting factor applied to your anchor text value. This makes a backlink from a higher PR page carry more anchor text value than a link from a low PR page.
Your anchor text value influences the relevancy score of your page for the targeted keywords within your anchor text. This in turn influences your ranking within the SERP. Your own page’s PR is used as a tie-breaker when you have the exact same relevancy score as another page. Therefore it is not important except in those rare cases when you have a tied relevancy score.
It would be much easier to improve your page relevancy than it would be to improve your PR and much better use of your resources. You can improve your page’s relevancy score by a combination of optimizing on-page content and building relevant backlinks from high PR pages.
So backlink value is rather difficult to measure and the PR of your own page is not that meaningful (except when selling links). You can have high PR with poor rankings or great rankings with low PR. Google doesn’t rank pages based on PR.