Problem in Calculated Column

Dear Friends,

I have created a summerzied table from another table,and now I faced a problem that how to implement a calculated columns.

As the below figure shows, First I would to add an index column which assign and index to the rows  based on the "Avg-position" value where their "cc","Type","Cluster,"Status" are the same. Then I would to make average on the  the first 20% minimum values of "Avg-Position" of each category. Hopefully my question is understandable.



Thank you,

Regards

Medi