If it's really an approximation it's one of the worst ones in history. http://en.wikipedia.org/wiki/Logarithmic_scale You can't just say PR 4 divided by 2 equals PR 2. Let's say PR 4 stands for 10.000 points and PR 3 stands for 4.000 points. Result: Two PR 4 links still result in PR 4. I hope that (made up) example made you understand what logarithmic scale means.