Rangesum Formula from Expression field

[Anonymous]

Hello!

I'd like to create a new dimension to get the sum  of the  Key Term Demand's  13 weeks and past due from an expression field...

                    Key Term                Past Due             Week1   Week 2   Week3 ------> Week 13

                    Demand                      10                       20            30               40            50

                    Supply                         15                         0            10              40               60

                    Forecast                      12                           10           40           10             70

In my sample above, I'd like to add the Demand from Past Due to week 13...The sum would then be divided into 13. Do i need toi use rangesum on this ?

Tks.