Local member issue

Hi Experts,

I have used local member for some calculation. Using this local member i want to perform some calculation and want to store the derived number. for ex:

Dec 14         Dec  14     Dec adj               2014.P13      Jan 15

Forecast        Actual      (Actual-forecast)  Forecast         Q4F

50                  100               50                    200              250(50+200)

From the above , Dec Adj is the local member with formula (Actual-Forecast). And i want to sum the Dec Adj number+2014.P13 and post into Jan15 and i want to save this Jan 15 number.

If user update the number for 2014.P13 then the jan15 will calculate automatically. Please help .

Please let me know if you require any other details. Thanks in advance.

Kind regards,

Srikant