Creating a calculated Benchmark within a dashboard

Hi all, 

I am trying to create a benchmark of % of spend for certain good types with Low Cost vs High cost countries such that I can filter my data set and see how much importer A currently imports from Low Cost Countries (and which) vs the benchmark of imports from low cost countries (and which)

I have a confidential data set that contains all of this information. I now just need to calculate the benchmark for low cost countries but I want it to be dynamic such that it always takes the 20th percentile as the benchmark. (i.e. if I filter for good A the 20th percentile benchmark may be 5% but if I filter for good B the 20th percentile benchmark may be 40%).

I hope that made sense.

Thanks,

Lucas