Calculating Churn Rate with Previous Month's User Base

How can I get a cohort's value in the previous month (or period) to use in a formula? My goal is to calculate churn rate, where:

Churn: total users who did the event "Churned" this month
User base at the end of the previous month.

The user base is defined as users who paid a subscription within the previous 45 days, as we offer 15 days tolerance after expiration. I'm currently using a cohort offsetting user base by 30 days, but that's not the same as EOM...