99.99%. What are your chances of surviving if your plane crashes into something like the world trade center? A few small holes (.45 or less) wouldn't even effect presurization. One flight had the whole top of the plane missing and landed safely. Only a stewardess was sucked out. All the passengers were fine.
Aloha Airlines, April 28, 1988. The roof from just behind the cockpit back to the wing blew off at 24,000 feet due to stress cracks in the fuselage. The plane made a safe landing at Maui, and suffered one dead, Clarabell Lansing who was the chief flight attendant, and 65 injured.