The fact that you'd use that number to automatically disqualify candidates is just insane. It doesn't give you a globally objective measure of a person's performance, because, like I stated, it depends on the competition in their particular group.

I could objectively be better at math than John from another school. But MY school is extremely competitive, so I'm probably in top 20%. But John doesn't have much competition in his school, so he might be in the top 10% in HIS school. But your system will reject me because I'm in the top 20%, and probably accept John because he's in the top 10%. Total absurdity!