Just for everyone’s info this is what worked
runningSum(
sum({WK-1 Abs Delta}),
[{Metric Date} ASC],
[{Reporting Week}, column_x]
)
In fact, you need to add the column you want it to reset in the last argument, so now the runnningSum continue for each metric date and resets at every reporting_week, column_x is the values you use as your row.