DAX VAR Function

Very difficult to troubleshoot without sample data. Please see this post regarding How to Get Your Question Answered Quickly: https://community.powerbi.com/t5/Community-Blog/How-to-Get-Your-Question-Answered-Quickly/ba-p/38490.

That being said, I would change your return to test the various parts of your formula, such as:

TEST = 
VAR CentreHeadcount = CALCULATE(COUNT('Centre Employees'[Employee ID]),'Calendar'[Month Offset]>0)
VAR CentreTerminations = CALCULATE(DISTINCTCOUNT('Centre Terminations'[Employee ID]),'Calendar'[Month Offset]>0)

return

VAR Terminations12MTD = CALCULATE(CentreTerminations,DATESINPERIOD('Calendar'[Date],LASTDATE('Calendar'[Date]),-12, MONTH),'Calendar'[Month Offset]>0)
VAR Headcount12MTD = CALCULATE(AVERAGEX(ALLSELECTED('Calendar'),CentreHeadcount),DATESINPERIOD('Calendar'[Date],LASTDATE('Calendar'[Date]),-12, MONTH),'Calendar'[Month Offset]>0)

return
Terminations12MTD
//CALCULATE(IF(DIVIDE(Terminations12MTD,Headcount12MTD)=BLANK(),0,DIVIDE(Terminations12MTD,Headcount12MTD)),'Calendar'[Month Offset]>0)

So, see if that value is BLANK and if it is that would explain why you are getting 0% back