The appellate system itself - of which cases involving new DNA evidence are a tiny fraction - is a much more useful measure.

There are a whole lot more exonerations via the appeals process than those driven by DNA evidence alone. This aught to be obvious, and the 0.2% provided by DNA is an extreme lower bound, not the actual rate of error correction.

Case in point, I found an article describing a study on overturning death penalty convictions, and they found that 7% of convictions were overturned on re-trial, and 75% of sentences were reduced from the death penalty upon re-trial.

One in fourteen sounds a lot more reasonable to me, and again that's just death penalty cases, for which you'd expect a higher than normal standard for conviction and sentencing.

The standard estimate is about 10% for the system as a whole.