Cumulative over rolling time period

Hi,

There has been a requirement from our client where we have to calculate a cumulative value on a given KF. How can it be made possible via back-end configuration?

For example, Say we have 1 KF: KF1@PL and another KF: CUMKF1@PL (This will be holding the Cumulative result of KF1). Now Let

KF1 have the following value over a 4 week period: 100     140     80     90

So CUMKF1 should contain:                                     100     240     320   410

Need help in this,

Regards,

Indaarjit