How to Calculate Your Customer Retention Rate 

Some customers at the end of the period “E” will have churned since “S”

100 at start. 10 Added. 95 at end. You've lost 15. Retention rate = 85%

If your E-N always equal S, then you have no leaving Customers. In real world, E-N usually less then S

This is the problem, it doesn't seem to be viable at except for specific businesses that very high frequent visits by customers or periodic subscriptions in wich you can use those periods to calculate otherwise it does always equal 1 because you have no way of knowing if that customer just hasn't needed a service that period.