Found this table yesterday - never seen anything like it - it tries to calculate how many links of each level of pagerank you need in order to reach what level of page rank for your site:
I've see either that table of another similar one before. Keep in mind that seo isn't about chasing PR though. Page rank is in the Google algorithm somewhere, but it's not nearly as important as a lot of people seem to think.
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i agree that PR is not that important rather than some other things...improve your site...focus on your site...optimize it for your visitors...make it a valuable, quality and trusted site...