DAX: Normalizing/Spreading out values over a month if condition is met

See if this works... I've created step by step measures so it's easier to see what the calculations deliver, though you could probably fuse it all into a single measure using variables.

Weeks by Month =
CALCULATE (
    COUNT ( 'Table'[Week ] ),
    ALLEXCEPT ( 'Table', 'Table'[Month], 'Table'[Item] )
)

First week value =
VAR _MinWeek =
    CALCULATE (
        MIN ( 'Table'[Week ] ),
        ALLEXCEPT ( 'Table', 'Table'[Month], 'Table'[Item] )
    )
VAR _Val =
    CALCULATE (
        SUM ( 'Table'[Value] ),
        FILTER (
            ALLEXCEPT ( 'Table', 'Table'[Month], 'Table'[Item] ),
            'Table'[Week ] = _MinWeek
        )
    )
RETURN
    _Val

Spread = 
DIVIDE([First week value], [Weeks by Month])

Final measure =
VAR _BOM =
    IF (
        SUM ( 'Table'[Value] ) = [First week value],
        [Spread],
        [Spread] + SUM ( 'Table'[Value] )
    )
RETURN
    IF ( MAX ( 'Table'[BOM] ) = "BOM", _BOM, SUM ( 'Table'[Value] ) )

If you prefer a calculated column (which is probably more useful in this instance), use:

Normalized =
VAR _weeks =
    CALCULATE (
        COUNT ( 'Table'[Week ] ),
        FILTER (
            'Table',
            'Table'[Month] = EARLIER ( 'Table'[Month] )
                && 'Table'[Item] = EARLIER ( 'Table'[Item] )
        )
    )
VAR _MinWeek =
    CALCULATE (
        MIN ( 'Table'[Week ] ),
        FILTER (
            'Table',
            'Table'[Month] = EARLIER ( 'Table'[Month] )
                && 'Table'[Item] = EARLIER ( 'Table'[Item] )
        )
    )
VAR _Val =
    CALCULATE (
        SUM ( 'Table'[Value] ),
        FILTER (
            'Table',
            'Table'[Month] = EARLIER ( 'Table'[Month] )
                && 'Table'[Item] = EARLIER ( 'Table'[Item] )
                && 'Table'[Week ] = _MinWeek
        )
    )
VAR _Spread =
    DIVIDE ( _Val, _weeks )
VAR _BOM =
    IF ( 'Table'[Value] = _val, _Spread, _spread + 'Table'[Value] )
VAR _FINAL =
    IF ( 'Table'[BOM] = "BOM", _BOM, 'Table'[Value] )
RETURN
    _FINAL

I've attached the sample PBIX file