Calculating churn is one of these things that there's a complete dichotomy on the web when it comes to the available information. You usually find very simplistic approaches to a quite complex problem and there's amazing research that is not easily accessible.

The article is going towards the right direction but as other commenters mentioned, churn is an example of a survival analysis problem.

A typical model used for modeling churn is pareto/nbd. Wikipedia is a great starting point https://en.wikipedia.org/wiki/Buy_Till_you_Die