Matrix Calculation on 1st and 2nd Level

Hello BOkungbure,

Can you please try this:

1. Calculate the Maximum Total across all Sections

Max Total Value = 
MAXX(
    ALL('YourTableName'[Main Section]),
    SUMX(
        FILTER(
            'YourTableName',
            'YourTableName'[Main Section] = EARLIER('YourTableName'[Main Section])
        ),
        'YourTableName'[Value]
    )
)

2. Calculate the Ratio

Ratio = 
VAR MainSectionTotal = 
SUMX(
    FILTER(
        'YourTableName',
        'YourTableName'[Main Section] = EARLIER('YourTableName'[Main Section])
    ),
    'YourTableName'[Value]
)

VAR CurrentValue = 
IF(
    ISINSCOPE('YourTableName'[Sub Section]),
    SUM('YourTableName'[Value]),
    MainSectionTotal
)

RETURN
DIVIDE(CurrentValue, [Max Total Value])

Should you require any further assistance, feel free to reach out to me.

Thanks Sahir,
This solution was super helpful, ISINSCOPE was the missing piece