Cumulative %

Cumul % = 
var cr = sum('Sales Table'[Qty])
var s = summarize(allselected('Sales Table'),'Sales Table'[Store],"q",sum('Sales Table'[Qty]))
var t = filter(s,[q]>=cr)
return divide(sumx(t,[q]),sumx(s,[q]))

Note that you have ties in your data so the result is a bit ugly.