On my experience the main problem is the final testing when you create large software projects, not the programming. If you have change something in your code you have to verify that alle basic features are still working. And i thing cuelux pro is complex. So for exampkle you took one day to fix a bug or to implement a feature and you need 5 days to test if everything else work as you want.
And when you find a problem you take a new turn. So good look that everything is finished to the deadline.