Reverse Running Total based On Another Column

FirstWeekToMeetTarget = 
var w = max('Table'[WIP])
return if(ISBLANK(w),BLANK(),
var wk = max('Table'[Date])
var a = CALCULATETABLE('Table',REMOVEFILTERS('Table'[SALES],'Table'[WIP]),'Table'[Date]<=wk)
var b = ADDCOLUMNS(a,"cm",var wkd = [Date] return sumx(filter(a,[Date]>=wkd),[SALES]))
var c = topn(1,filter(b,[cm]>=w),[Date],DESC)
return CONCATENATEX(c,[Date])
)

You should be able to add that yourself. filter all week values to include the interval from the FirstWeekToMeetTarget to the current week, and return the count of rows.  Minus one if you don't want to count the current week.