How to get previous month's values for current month

Hi..

I have a requirement on a report, where for a particular month, the key figure value should be from the previous month.

Eg: KF1 value for Sept 2007 should come from Aug 2007, KF value for Dec 2007 should be the value of Nov 2007, etc.

There are other key figures on the report which do not need this logic. It is only for one key figure.

Should we do this on the backend or front-end, and how? Any tips will be appreciated.

Thanks

R.