The % Done you see just below the Status field in the Edit Project Information page is the same as the Estimated Average % Done which may be displayed in the  Home => Projects > Projects page. This is also the value used in the project Gantt charts to display overall progress on the projects.

This overall project progress is a weighted average (which means that a task with many hours weighs more heavily in the result than another with fewer hours). Here is the formula used to calculate this average:

Estimated Average % Done = (Sum (Task Estimated Hours x %Done)) / Sum (Task Estimated Hours)

Here’s a simple example:
A project has two tasks: task 1 has 10 Estimated Hours and is currently at 80% done task 2 has 20 Estimated Hours and is currently at 20% done

Estimated Average % Done = (Sum (Task Estimated Hours x %Done)) / Sum (Task Estimated Hours)
Estimated Average % Done = (10 hours x 0.8 + 20 hours x 0.2 ) / (10 hours + 20 hours)
Estimated Average % Done = ( 8 hours + 4 hours ) / (30 hours)
Estimated Average % Done = ( 12 hours ) / (30 hours)
Estimated Average % Done = 40 %

Now, if the estimated hours for each task in the project is 0, the denominator in the previous formula is then equal to zero. Dividing by zero is mathematically undefined, so AceProject simply returns 0% in this particular situation. Hence, it is important to provide values (other than 0) for the Estimated Hours field on your tasks for your project progress percentage to properly reflect progress. You can even set a default Task Estimated Hours value for all future tasks in the project under  Project Tab > Edit Project  (in the Default Values section of the page). This tip will be particularly useful if your organization does not need to specify estimated hours on tasks.