Everybody’s definition of churn is different, even within the same business.
My recommendation for starting the conversation around churn measurement is to first figure out why it’s important to measure that number at your company right now. Get that story crisp. Then, start with your definition based on an analysis of the raw data and a generic textbook definition with either month/month or year/year, depending on your business model. Then, head to the table and start the conversations.
More likely than not, your first iteration will change, a lot. That’s okay – don’t take it personally. By the end of this process, you’ll have gone through anywhere from three to fifteen iterations.