Determine context value dinamically

Hi Gurus,

I have to develop a report where as input value the user sets the year and in the columns we have the calendar week and in rows several KFs.

Let say we have KF1 and KF2. KF1 is a ratio from the underlying infoprovider, Sales volume. KF2 is the average of KF1 for the last 4 weeks(not including the one in context).

So my concern is how could I determine dinamically the value for the week in the context of the calculation so I can get the values for previous weeks. Here you have an example:

Calendar week 2 3 4 5 6 ......

Actual Sales 200 300 400 500 250

Avg. actual sales

last 4 weeks ((200 + 300 + 400 + 500) / 4) = 350

Thanks a lot in advance!