So I am working on a problem where we are trying to assess the influence of a case via its Cook's distance. In the text, there is an example:

"To assess the magnitude of the influence of case 3 (D₃ = 0.490), we refer to the corresponding F distribution, namely, F(p, n - p) = F(3,17). We find that 0.490 is the 30.6th percentile of this distribution."

...how in the world did they figure out that 0.490 is the 30.6th percentile of that distribution? I've worked with F tables before, but what/how did they do that? I have no clue. I can't find a calculator or any other resource that can help me.

Please, help me!!