Headroom? There's no headroom in datasheets!
Drop out. Its called Drop out. and this chip is called a LOW DROP OUT regulator.
The chip claims ~30-50mV drop out under ideal conditions and 500mv under worst conditions...
So to get 3.3V out you only need to supply 1/2 a volt more for "typical" current draw... but of course its not a perfect world, so I would round up and say 4.3v is worst case (or best case if you are a glass half full person) and should be a design goal. It is easily met by the 5V regulator.