Scott’s Rule of Documentation Efficiency

How well is the documentation for a product written? What is the measure of “documentation efficiency”?

Given a problem with trying to use a product feature, what is the probability that you can find the answer to the problem by looking in the documentation, without having to search through stackoverflow?

If the percentage is less than 50%, it means you are better off searching through stackoverflow first, before using the documentation. The documentation efficiency is, thus, not good.

P.S. Scott is one of our engineers.