> Suppose that a website only has links from PR X sites. How many such
> links would it take to get to level X+1?
The PR system is not linear
so the question has to be rephrased. To climb from PR0 to PR1, a few weak
links would do. To move from PR6 to PR7, these links are negligible and
more links of the same 'level' are probably needed.
The main point is that PR cannot be explained by that formulation above. You
have more than a single parameter to tweak.
Roy S. Schestowitz