Merge Data in a filter table

Hi LukeA ，

Have you tried to create measures or columns to get what you want? Such as:

Sum - Measure =
CALCULATE (
    SUM ( 'Table'[Stock on Hand] ),
    ALLEXCEPT ( 'Table', 'Table'[Month], 'Table'[Size], 'Table'[Item name] )
)

Sum - Column =
CALCULATE (
    SUM ( 'Table'[Stock on Hand] ),
    FILTER (
        'Table',
        'Table'[Month] = EARLIER ( 'Table'[Month] )
            && 'Table'[Size] = EARLIER ( 'Table'[Size] )
            && 'Table'[Item name] = EARLIER ( 'Table'[Item name] )
    )
)

If this isn't what you want, it would be better to share a sample .pbix file for test. Please don't contain any sensitive information.

Reference: How to provide sample data in the Power BI Forum - Microsoft Power BI Community

Best Regards,

Icey

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