There are three options to calculate project percentage completion.
Based
on task and issue count: The project completion percentage is
calculated based on the Open and Closed Status of both tasks and Issues.
Based on task percentage completion and Issue completed: The project completion percentage is calculated based on each task completion percentage. If there is no percentage completion in the issue module, status type (Open,Close) will be considered. In this type all tasks and issues will have equal weightage (Weightage = 1).
Based
on Task and Issue weightage field: Users can customize the weightage for each task
and issue. The project completion percentage is calculated based on the
weightage value given in weightage field (numeric field or task duration
or work hours).
Select you default option and click Update.