O1L,
It is not just the percentage of the hole, the size of the hole is also critical. A simple experiment will show this. Blow an electric fan through a window screen, which I am guessing is at least 75% open, and see how much air passes through. The faster the air moves, the less it will pass through a small opening. I am one of the many that has found out the hard way that 1/4" screen on the front of a racecar at 100+MPH will overheat in no time, it has to be 3/8" or better. I know this sounds absurd, but when a car overheats constantly for no apparant reason, take the screen off and see what happens. Numerous people more knowlegable than me on the subject have verified this after the fact.