Where to find the numbers
Your platform shows which subscribers have auto-renew turned off. Count the ones whose subscription ends in the next week or two. For other spend, use what these fans have spent per month recently, not your average across all fans.
Using the result
The monthly figure is the size of the problem; the share you keep is the part you can influence. A short, personal message before the subscription ends, written and sent by you, is usually the first thing to try.