Project completion is based on the Completed Work Hours based on percent complete values of each task's work hours.
So, for example, if you have three tasks:
- Task A is 2 hours
- Task B is 30 hours
- Task C is 8 hours
- and Task A is 100% complete
- and Task B is 25% complete
Your project is 24% done because Task B is so much bigger than the other two tasks.
[Math = (2+7.5 accomplished hours = 9.5) / 40 total hours to get done on the whole project]