formula-based metrics

[frog]

Hello...I create some KPIs in a Metric Studio project. Values will be loaded by users or using tab-delimited files.I would need then to create a KPI which is the 'running sum' over the months of one of the KPI defined. For example KPI1 has values 10 for January, 10 for February. My new KPI, that has to be automatically calculated, should have values 10 for January, 20 for February and so on...Any hint???
Thanks a lot!