Generate parent id from BOM level information

Zubair_Muhammad Thank you, that DAX suggestion works, but I could not get the Power Query one to work. It only generates null values.

Now I have two working solutions to this:

Parent = 
MAXX(
    TOPN(
        1;
        FILTER(
            'BOM';
            'BOM'[Level] = EARLIER('BOM'[Level]) - 1 &&
            'BOM'[Index] < EARLIER('BOM'[Index])
        );
        'BOM'[Index]; DESC
        );
    'BOM'[Index]
)

and

Parent = 
CALCULATE(
    LASTNONBLANK(VALUES(BOM[Index]);0);
    FILTER(
        ALL(BOM);
        BOM[Level] = EARLIER(BOM[Level]) - 1 &&
        BOM[Index] < EARLIER(BOM[Index])
    )
)