I agree that a pilot doing only 90 minute flights would have a lower annual exposure, but what about A330 pilots doing lots of PER to MEL/SYD flights with 4 hours in cruise at FL390-410 at higher latitudes? I suspect their annual exposure would be more than doing a year's worth of 'project sunrise' flying across the equator at FL320-380. All of these rosters are going to be well below the annual 20 mSv limits anyway.