Sales PM :=
VAR CurrentYearMonth = SELECTEDVALUE ( 'Date'[Year Month Number] )
VAR PreviousYearMonth =
    CALCULATE (
        MAX ( 'Date'[Year Month Number] ),
        ALLSELECTED ( 'Date' ),
        KEEPFILTERS ( 'Date'[Year Month Number] < CurrentYearMonth )
    )
VAR Result =
    CALCULATE (
        [Sales Amount],
        'Date'[Year Month Number] = PreviousYearMonth,
        REMOVEFILTERS ( 'Date' )
    )
RETURN
    Result

Sales Diff PM :=
VAR SalesCurrentMonth = [Sales Amount]
VAR SalesPreviousMonth = [Sales PM]
VAR Result =
    DIVIDE (
        SalesCurrentMonth - SalesPreviousMonth,
        ( NOT ISBLANK ( SalesCurrentMonth ) ) * ( NOT ISBLANK ( SalesPreviousMonth ) )
    )
RETURN
    Result

% Sales Diff PM =
DIVIDE (
    [Sales Diff PM],
    [Sales PM]
)