Your best bet is probably to use the Data Explorer. I spent just a few seconds on this, so it's awful, but perhaps a starting point:
select count(*) from (
select OwnerUserId, sum(Score) as Total from Posts
where tags like '%android%'
group by OwnerUserId
having sum(Score) > 8
) as foo
(From your profile you currently have a score of 8 in the Android tag.)
It returns 2186 results, and if you remove the HAVING clause it returns 32054. That would put you in the top 7%, so it's clearly wrong :P. Top 20% might be based on rep in the tag rather than the score in the tag, or maybe score on answers only, I can't find any info on it, or perhaps my SQL is wrong. But again, it's just a place to start.