Rolling calculation

I have a sample file attached here. My objective is to get rolling calculation.

Product A is budgeted for 6 month sales. Against each month actual sales are populated. Assuming we have ended Feb'08, I want to calculate in cell O4, rolling forecast amount which will be actuals upto feb'08 i.e. (50 + 60) + budgeted for March'08 to June'08 i.e.( 70 +70 +75 +75)= 400.

Basically the command/ string should be dependent on change of month in particular cell , where if I put month end as Jan-08, then rolling forecast will be actuals of Jan'08 + budgeted from Feb'08 to June'08.

thanks in anticipation

sorry forgot the attachment earlier !