Staff Turnover - Need Help

Hi Siddiq8686 
Here is the file with the solution https://www.dropbox.com/t/VXuA2561R4sF1Pul
The data model looks like this

The flag calculated column in the Merge_File3 table is

Quarterly Population = 
IF ( 
    MONTH ( Merge_File3[Payroll Process Date] ) IN { 1, 4, 7, 10 },
    TRUE 
)

The measures are

Average Population = 
VAR MonthlyPopulation =
    SUM ( Merge_File3[FTE] )
VAR QuarterAveragePopulation =
    AVERAGEX ( 
        VALUES ('Calendar Table'[Year Quarter] ),
        CALCULATE ( 
            SUM ( Merge_File3[FTE] ),
            Merge_File3[Quarterly Population] = TRUE
        )
    )
VAR Result = 
    IF (
        HASONEVALUE ( 'Calendar Table'[Year Month] ),
        MonthlyPopulation ,
        QuarterAveragePopulation 
    )
RETURN
    Result

Employee Left = COUNTROWS ( Separation_Data4 )

Employee Left RT = 
VAR LastMonthInFilter =
    MAX ('Calendar Table'[Year Month Number] )
RETURN
    CALCULATE ( 
        [Employee Left],
        REMOVEFILTERS ('Calendar Table' ),
        'Calendar Table'[Year Month Number] <= LastMonthInFilter
    )

% Turnover = 
DIVIDE (
    [Employee Left],
    SUMX ( 
        VALUES ('Calendar Table'[Year] ),
        [Average Population]
    )
)

New Hires = COUNTROWS ( Hireing_Data )

Hires RT = 
VAR LastMonthInFilter = 
    MAX ( 'Calendar Table'[Year Month Number] )
VAR Result =
    CALCULATE ( 
        [New Hires],
        REMOVEFILTERS ( 'Calendar Table' ),
        'Calendar Table'[Year Month Number] <= LastMonthInFilter
    )
RETURN
    Result

Total Population = [Hires RT] - [Employee Left RT]

Your report looks like this

Please let me know if you still have any doupt.