Hi Mike, one immediate thought is to right click on your Map sheet tab, and choose Duplicate as Crosstab. See how everything is playing together in a table, get the numbers in the table as they should be, then go back and apply this logic to your Map. Sometimes it is easier to see how to solve a problem when the numbers are laid out for you in tabular form.
Here's a mock up of a way to achieve this sort.
I have created 2 parameters, one controls the Top/Bottom N and the other controls whether you wish to see the top or bottom (pretty standard set up).
Next a created a calculated field:
This will change if the sort is ascending or descending.
Last step is to create a set:
Add the set to the filters and you now have a switchable map!
I hope this helps!
Workbook is attached
Top & Bottom Sort.twbx 1.2 MB