Input template worksheet logic requirement

Hello,

I need to accomplish the following requirement that I need some help with. Lets say the user selects Forecast_Vmay2012, the may 2012 value of a specific account ABC should sum Jan-Apr 2011 XYZ Account values. This needs to be flexible, in other dimension member Forecast_vmay2012 has current month property with value of 5. Just like that, Forecast_Vaug2012 has value of 8 etc. based on this, the worksheet logic needs to sum previous year's initial current month -1(in our initial example current month value is 5, so sum 1st 4). Any idea on how to accomplish it?

Thanks.