Average Calculation from Actual to Plan

Hi all

I have a requirement of Average calculation from Actuals to plan.I am looking TMVL function to calculate and write to the Plan category in data base and I can do with using fixed year and periods but not dynmic selections, Looking for your inputs / guidence and application model is YTD and Periods were JAN to DEC.

Ex:

Jan

Act 1 Actuals $ 1000

Average Calculation = Value/no of Months i.e. = 1000/1 = 10000

Feb

Act 1 Actuals $ 3000

Average Calculation = Value(Jan+Feb)/no of Months i.e. = 4000/2 = 2000

Appreciated your help

Thanks

Ramana