I always wondered if they anticipated the presence of low deck of scattered clouds (around 1000 ft) above the water level. You can't really spot such tiny island when you are above them but if you go below the clouds the horizon is limited (the radius is about 60 km from that altitude, probably closer to 50 km if you want to spot something). But Noonan flew a lot in that area for Pan Am (if I am not mistaken) and he must have been aware of the typical weather patterns there. A guy with his navigation credentials could calculate the maximum error of his navigation procedure - this way he could have estimated probability of spotting or missing the island even before the trip. Probably someone well acquainted with navigation method at the time could perform such calculation today, it could be very educational and would shed light how much pure 'luck' was a factor there, I never saw it done. This of course assumes they they worked perfectly as a team, that Amelia never doubted her navigator which we know is a stretch.