Hey Guys (& Ladies), I am wondering if anyone knows, assuming a model exists that allows you to figure out how to achieve a certain PR. For example: 3 PR5 Links to your website = YourSite.com a PR3 Cheers!
Google had done completely changed on PR and no one could give you exact metric to calculate PR these days, thus the PR calculation is not that simple like you mentioned anyway. If you are still interesting I would recommend you read this sources. Please be aware that it may not up-to-date but it could help you illustrate. Google PageRank calculation Google's PageRank Explained
It has changed over the years but is still basically the same. The PR passed from one website to another depends on PR of the page where your link is on and how many links are on the page. (both internal and external links)
I don't think there is a specific metrices out there that can determine what PR you can get. Your best bet is just to go for high-quality PR site links.