100% accuracy
If it were a truly effective deterrent, I could live with best effort to prevent mistakes. That should be an even better effort than we have seen so far, I might add.
There is no such thing as certainty. The execution process takes a long time and wrongfully imprisoning someone for years (essentially destroying their life) is not a lot better than wrongful execution.
Someone here (maybe MA) once said that if certainty is a proper test we would have to look at a lot of things we do such as commercial flying.