Find first value above 50% by category using DAX

Hi,

I am not sure if I understood your question correctly, but please check the below picture and the attached pbix file.

I tried to create a sample pbix file like below, and I hope the below can provide some ideas on how to create a solution for your datamodel.

Income: = 
SUM( Data[Income] )

Income cumulative ratio: = 
VAR _runningtotal =
    CALCULATE (
        [Income:],
        WINDOW (
            1,
            ABS,
            0,
            REL,
            ADDCOLUMNS (
                SUMMARIZE ( ALL ( Data ), PostCode[PostCode], Data[Customer] ),
                "@income", [Income:]
            ),
            ORDERBY ( [@income], ASC ),
            KEEP,
            PARTITIONBY ( PostCode[PostCode] )
        )
    )
VAR _allincomebypostcode =
    CALCULATE ( [Income:], ALL ( Data[Customer] ) )
VAR _cumulativepercentage =
    DIVIDE ( _runningtotal, _allincomebypostcode )
RETURN
    _cumulativepercentage

First value above 50%: = 
IF (
    [Income cumulative ratio:]
        = MINX (
            FILTER (
                ADDCOLUMNS ( ALL ( Data[Customer] ), "@ratio", [Income cumulative ratio:] ),
                [@ratio] > 0.5
            ),
            [@ratio]
        ),
    [Income:]
)