Calculate balance in all month from opening and period sales

Dear Experts,

Sorry for disturbing you.

I am new in IP and i have requirement to to create a planning function should calculate balance in all month from
opening and period sales.

In the aggregation level i have only 0sales (Sales for period) and in the cube i have opening Balance ZOPENBAL and cumulativ Balance 0BALANCE.

Could you please help me by providing me the appropriate code?

Do you have a doc or a book which could help to learn FOX?

Thank you in advance

Pat.