Answer

Option 1

1. Select Analysis > Create Calculated Field with a formula similar to the following: 

   INDEX() <= INT(SIZE()/5)

2. Place the calculated field onto the Filters shelf and select True.

Note: Dividing by 5 will give the top 20 percent. This number can be adjusted to achieve the desired percentage. 

Option 2

1. Create a parameter Top N% as a float between 0 and 1, step size 0.01. Optionally, format the display format to 'Percentage'
2. Create a calculated field 'Top N percent' that returns the n-th percentile value of the data set:
    PERCENTILE([Sales],[Top N%]) 
3. Create a calculated field 'Top N Filter' that filters out the Top N percent:
4. [Sales] <= [Top N Percent]
5. Use the field 'Top N filter' on the Filters shelf, selecting only 'True'.