You were correct when you said there isn't math for that sorta thing. You can have 50k backlinks, and still be a PR 0.
You also don't have to have a higher page rank to out rank other sites. In other words, measuring your PR is not a good indicator of how you are going to rank.
Consider this too - a site might have a high PR and tons of links, but if they aren't optimized for a specific keyword, then how much are they really worth?
If you want to rank for "pocket knives" and pocketknives.com has a PR of 6, then yeah, that's hard. But if it's Walmart is at number 1 with a PR of 6 but no title/description tags and no backlink anchor that say "pocket knives", then their authority doesn't mean as much for that keyword.