Max Faget’s position was that considering the difficulty of the job,  if each mission was successful half the time, it would be well worth the effort.  But Gilruth thought that was too low.  He want a 90% mission success ratio and a 99% ratio for Astronaut safety.  Walt Williams who was currently running the Mercury program believed that astronaut safety needed to be limited to only 1 failure in a million which was 99.9999%.