I have seen this stat often on this site and I understand what it is and its purpose but what I don't know that I want to know is how to calculate it. I would love to be able to calculate it myself and I'm sure other people on the site would as well.

I found this explanation on how to calculate it but the way it is explained is a little over my head as it looks like how you would do it on an Excel spread sheet and I do not know how to use excel. If anyone can explain to me how to do it that would be great.