calculate average headcount & attrition

Hi Guys, 

I'm trying to calculate average headcount per month, it's part of a formula to calculate attrition. 

the end result is I would like to see is a line chart showing  average headcount for each month. 

For attrition I would like to see a line chart showing Attrition in % for each month. 

I have been trying to get this working for a while. unfortunately no luck for me. 

If someone is up for the challange I have uploaded PBIX file with dummy data here. 

attrition formula I'm trying to get to is this: (YTD Unmanaged Leavers / FY month we are in * 12) / YTD AVR Headcount

Thanks to all who has helped me in the past