replicate Excel SLOPE calculation

Hi, Please check the below picture and the attached pbix file.

Volume total: = 
SUM( Data[Volume] )

WINDOW function (DAX) - DAX | Microsoft Learn

Slope: = 
VAR _calendar =
    ALL ( 'Calendar'[Year-Month], 'Calendar'[Year-Month sort] )
VAR _window =
    WINDOW (
        -11,
        REL,
        0,
        REL,
        _calendar,
        ORDERBY ( 'Calendar'[Year-Month sort], ASC )
    )
VAR _condition =
    COUNTROWS ( FILTER ( _window, [Volume total:] <> BLANK () ) ) = 12
VAR _t =
    ADDCOLUMNS (
        _window,
        "@x", RANK ( SKIP, _calendar, ORDERBY ( 'Calendar'[Year-Month sort], ASC ) ),
        "@y", [Volume total:]
    )
VAR _linestx =
    LINESTX ( _t, [@y], [@x] )
RETURN
    IF ( _condition, MAXX ( _linestx, [Slope1] ) )