You might take a look at this knowledge base article for how to use Sets (instead of Index()) to accomplish top/bottom: Creating Sets for Top N and Others | Tableau Software. The benefit of this approach is that it is much simpler and keeps the top/bottom as a row level calculation so it will be much easier to use additional calculations (row, aggregate, or table) -- it should be fairly easy to get the sum. Also, you should be able to get both Top and Bottom in a single chart (using a combined set of Top and Bottom).
If you have any issues, please post a packaged workbook and I'd be happy to take a look!
Thanks for the article Joshua!
It helped a bit and I can now find the top x performers based on a field but I still dont know how to make calculations based on the set. What I want is a chart with one column for a single customer of my choosing and then the sum/avg/whatever of the top X customers next to it. I do not want to have a column for each member of the set, I want an aggregated column for the set. Is that possible?
Could the selected customer of your choosing be a member of the top X customers or not? The approach might need to change based on that.
For an idea of how to use sets in calculations, see http://www.tableausoftware.com/about/blog/2013/4/lets-talk-about-sets-23043.
I have a similar problem in getting the percentage of any selected customers (not top n nor bottom n) compared to the total customers. Grouping seems works but not dynamic in selecting customers, any suggestion?
Are you looking to select a single customer (or have a row for each customer), or to bundle the selected customers into a group comparison vs. total?