To get the TOTAL of the Top N values based on the Index function

Hi,

In my workbook I am creating the TOP 20 customers based on the spend( The highest spending customer as RANK 1...).

I have created this by using the INDEX function, I have created a calculated field Index() and then selected the top 20 values.

Now my requirement is to get the TOTAL of the spend of only TOP 20 customers, But when I am using the Grand Total function, it is showing me the Total of all the customers.

Please assit how can I achieve the total of only Top 20 0 customers?

REgards,

Mehak