If you are using Percent Complete to update your task status, you might want to use the Update Tasks dialog. This dialog contains the fields you need to update to properly use Percent Complete as your status update method.
Understanding the Update Tasks Dialog
Click the Update Tasks toolbar button on the Tracking toolbar.
Enter your Percent Complete value here.
Enter a value for the Actual Duration field that has been used on this task so far, if you so desire .
Edit the Remaining Duration if you have an estimate for how much longer the task will take AFTER the Actual Duration value you just entered is taken into account.
You can also enter the Actual Start and or Actual Finish values for the task.
Click OK when you are finished editing the fields.
For Your Information
Advantages of the Update Tasks Dialog
One advantage to using this dialog is that none of the edits to the fields are applied to the task until you click OK. Editing these fields in the table can cause some confusion because Project will recalculate some fields as you enter values. The "all at once" way in which this dialog edits the fields is more predictable for new users.
Updating Status Using the Update Tasks Dialog
Select the task you want to update.
Click the Update Tasks button on the Tracking toolbar.