Staff Turnover - rolling 12 month

[Aladdin777]

https://www.dropbox.com/s/cqthca8uxpvis77/HC_Testfile.pbix?dl=0 

Posted this previously but received no response.

This time have attached link to pbix.

As per example need to calculate sum(12 months leavers)/average(12 months total staff).

Needs to work if I want to filter by department too.

Any help would be appreciated.