Subscriber churn metric

I want to see the churn rate for subscribers to our list. The formula is:

 (Total Lost Subscribers / Subscribers in the Period) x 100

Apparently the only way I can do this in OP now is to create a metric that calculates all of the new contacts that are created within a certain time frame. Then, create a second metric for contacts who've had their bulk email status updated to transactional or hard bounced in that same time frame. Then, manually plug those numbers into the equation above.

This is very manual. Churn is a standard metric used by sales / marketing all over the world. Would be great as a standard metric. Thanks