Sum previous rows in a new column

HI c0ld,

Account to your description, it sounds like a cumulative calculation between two fields. Do any Index fields exist in your tables?  If that is the case, you can refer to the following DAX formula to get results.

Measure:

formula =
VAR currIndex =
    MAX ( Table[Index] )
VAR rollTP =
    CALCULATE (
        SUM ( Table[TP] ),
        FILTER ( ALLSELECTED ( Table ), [Index] <= currIndex )
    )
VAR rollFN =
    CALCULATE (
        SUM ( Table[FN] ),
        FILTER ( ALLSELECTED ( Table ), [Index] <= currIndex )
    )
RETURN
    DIVIDE ( rollTP, rollTP + rollFN )

Calculated column:

formula =
VAR rollTP =
    CALCULATE (
        SUM ( Table[TP] ),
        FILTER ( Table, [Index] <= EARLIER ( Table[Index] ) )
    )
VAR rollFN =
    CALCULATE (
        SUM ( Table[FN] ),
        FILTER ( Table, [Index] <= EARLIER ( Table[Index] ) )
    )
RETURN
    DIVIDE ( rollTP, rollTP + rollFN )

Regards,

Xiaoxin Sheng