At the task level, the % Completed indicates the portion of the task that has been completed in terms of duration time (not effort). It is computed as follows:
% Completed = (Completed duration / (Completed Duration + Remaining Duration)) x 100
When the % Completed entered is greater than zero, the Start date of the task is automatically copied into the Actual Start field. The task then becomes anchored to this date and cannot be moved by other scheduling constraints.
Prior to entering any % Completed, an Actual Start date should always be entered first. The reason for this is that the modification of the Actual Start date is only possible when % Completed is zero.
At the project level, the % Completed field indicates the portion of the project that has been completed in terms of duration (not effort).
The % Completed value is taken into account in chronological order. If a task is skipped, the % Completed will be calculated up to the previous task, even if some % Completed has been entered for Tasks whose dates fall after the skipped task.