Webi Variable to cumulate data based on Start week and End week

Hi Experts,

Could you please help in below scenario,

I have APO Week, Calender Week and a Keyfigure ( Absolute  measure), As attached in pic

And I have Lag period which user inputs from which I can derivie Start Week of Calender and end week of calender dynamically.(This is not fixed)

User may enter any lag period.If user gives Lag period 2, I need to consider for calculations Start week will be 21.2014 and End week will be 22.2014.

In the below example  I have enter lag Period as 3, So while cumulating Measure I have to consider 3 weeks i.e 21.2014,to 23.2014 ,

Could you help me how can I achieve add measure for those weeks,

Even I I have created variable  like below, but still not working

=[ABS (Fcst - (Actual + OOS))] Where ([Calendar year / week] Between ([ABS Start week];[ABS End week]))

In my report output I do not need Calweek., Only I need APO week

Thanks and Regards

Sree