Rolling 12M Average measure

Thank you all for your replies. I was playing around and found this to give me what I needed:

_HC = 

VAR _MaxCalendarIDInData =
    CALCULATE(
        MAX('Employee headcount'[CalendarID]),
        ALL('Employee headcount')
    )
 
// Convert CalendarID back to Date for calculations
VAR _MaxDateInData =
    CALCULATE(
        MAX('Calendar'[Date]),
        FILTER(ALL('Calendar'), 'Calendar'[CalendarID] = _MaxCalendarIDInData)
    )
 
 
// Detect Calendar filtering
VAR _IsCalendarFiltered =
    COUNTROWS(ALLSELECTED('Calendar')) < COUNTROWS(ALL('Calendar'))
 
// Raw max date from selection (may be in the future)
VAR _SelectedMaxDate =
    IF(
        _IsCalendarFiltered,
        MAX('Calendar'[Date]),
        _MaxDateInData
    )
 
// HARD CAP: never go beyond last date in Employee headcount
VAR _MaxDate =
    MIN(_SelectedMaxDate, _MaxDateInData)
 
// For rolling 12-month window
VAR _MaxDateProjectedToMonthEnd = EOMONTH(_MaxDate, 0)
VAR _MinMonthFor12Months = EDATE(_MaxDateProjectedToMonthEnd, -11)
 
// Get distinct dates that exist in Employee headcount table within 12-month window
VAR _AvailableDates =
    DISTINCT(
        SELECTCOLUMNS(
            FILTER(
                'Employee headcount',
                RELATED('Calendar'[Date]) >= _MinMonthFor12Months &&
                RELATED('Calendar'[Date]) <= _MaxDate &&
                'Employee headcount'[EmploymentIsTerminatedIn1/0] = 0
            ),
            "Date", RELATED('Calendar'[Date])
        )
    )
 
// Build month list from available dates only (last 12 months)
VAR _MonthStarts =
    DISTINCT(
        SELECTCOLUMNS(
            _AvailableDates,
            "MonthStart",
                DATE(
                    YEAR([Date]),
                    MONTH([Date]),
                    1
                )
        )
    )
 
VAR _AvgHC =
    AVERAGEX(
        _MonthStarts,
        VAR _MonthStart = [MonthStart]
        VAR _MonthEnd = EOMONTH(_MonthStart, 0)
        RETURN
            CALCULATE(
                SUM('Employee headcount'[Count]),
                'Employee headcount'[EmploymentIsActiveIn1/0] = 1,
                'Employee headcount'[EmploymentIsTerminatedIn1/0] = 0,
                'Employee type'[Employee type name] IN
                {   
                    "Apprentice",
                    "Blue-collar Employee",
                    "Expatriate",
                    "Indirect Blue-collar Employee",
                    "Trainee",
                    "White-collar Employee"
                },
                NOT 'Employee status'[Employee status name] IN {
                    "Reported No Show",
                    "Retired",
                    "Terminated"
                },
                'Calendar'[Date] >= _MonthStart,
                'Calendar'[Date] <= _MonthEnd
            )
    )
 
RETURN
IF(_AvgHC >= 4, _AvgHC, BLANK())