I ran into this problem again with a Project Server 2007 customer today.  The project plan had more than 2000 lines and the % Complete at the Project Summary Task level did not make sense to the project manager; it seemed much lower than it should have considering the work already done on the plan.  This is because of how the % Complete is calculated for summary tasks.  The apparent discrepancy is compounded when you have many summary and sub- tasks.

Here’s the article you will need to explain to anyone how the % Complete field is calculated.  It’s old, but pertinent even for the most recent version of Project, and was provided by Microsoft Customer Technical Support.

      “How Percent Complete Is Calculated for Nested Summary Taskshttp://support.microsoft.com/kb/101495/EN-US

You may also consider setting calculations to be automatic in Project 2007 using the instructions below.  This won’t change how the % Complete field is calculated but will show how changes to the plan affect the plan automatically.  Also ensure there are no resources assigned to summary tasks at any level and no tasks are successors to the project summary task.

To set the calculation mode in Project Pro 2007 to automatic using below steps.

      1. Open Project Professional
      2. Select Tools > Options>
      3. Select Calculation Tab.
      4. Set the Calculation Mode to Automatic.