Average headcount (current + previous month)/2

Hi,

I am not sure how your semantic model looks like, but I tried to create a sample pbix file like below.

Please check the below picture and the attached pbix file.

Count avg: = 
SUMX (
    SUMMARIZE ( 'Calendar', 'Calendar'[Month-Year sort], 'Calendar'[Month-Year] ),
    CALCULATE (
        VAR _currentmonth =
            SUM ( data[count] )
        VAR _prevmonth =
            CALCULATE (
                SUM ( data[count] ),
                OFFSET (
                    -1,
                    ALL ( 'Calendar'[Month-Year], 'Calendar'[Month-Year sort] ),
                    ORDERBY ( 'Calendar'[Month-Year sort], ASC )
                )
            )
        RETURN
            IF ( _currentmonth && _prevmonth, DIVIDE ( _currentmonth + _prevmonth, 2 ) )
    )
)