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If we were to independently create a measure for new customers without already having a returning customers measure, it would look like this:

New Customers Alt = 
    VAR __Month = MAX('R02_Table'[Month])
    VAR __Year = MAX('R02_Table'[Year])
    VAR __PreviousCustomers = 
        DISTINCT(
            SELECTCOLUMNS(
                FILTER(
                    ALL('R02_Table'), 
                    [Date] < DATE(__Year,__Month,1)
                ),
                "__Customer",[Customer]
            )
        )
    VAR __CurrentCustomers = 
        DISTINCT(
            SELECTCOLUMNS(
                'R02_Table',
                "__Customer",[Customer]
            )
        )
    VAR __NewCustomers = EXCEPT(__CurrentCustomers, __PreviousCustomers)
    VAR __Count = COUNTROWS(__NewCustomers)
RETURN
    IF( ISBLANK(__Count), 0, __Count)