Calculate cumulative sums and obtain highest value based on it

MMULT necessary since OP's Month column is YYYY MMM text. If only it were date values (numbers) just formatted like that, then it'd be possible to use a shorter formula

=MAX(SUMIFS(Table1[Data1],Table1[Agent],Table1[Agent],Table1[Agent],"<>TOTAL",Table1[Month],"<="&--(J1&H1)))

Originally Posted by hrlngrv

MMULT necessary since OP's Month column is YYYY MMM text.

Yes and the options #3 ,№4 based on Eng regional settings ( --(J1&H1) ).
But i see "It is preferred if no additional helper columns are used. The concatenate column already exists in the original table, and can be used if needed." in the file.