Far less than 1%.
And the vast majority of overturned convictions are overturned on technicalities - custody of evidence, improper mirandizing, procedural flaws, etc. - not because the defendant was innocent.
And, as Robert Morgenthau showed us in the Central Park rape incident, people who are totally guilty can be proclaimed innocent after the fact - in other words, some of those overturned convictions were falsely overturned.
Apparently you watch far too much TV. My wife was a criminal attorney and she'll tell you that Law and Order is just a TV show.