By default, Microsoft Office Project defines a task as critical if it has zero days of slack. However, you can change this definition and make a task critical if it has, for example, one or two days of slack.
On the Tools menu, click Options, and then click the Calculation tab.
In the Tasks are critical if slack is less than or equal to box, type or select the number of slack days. If the number of slack days for a task is less than or equal to the number that you specify here, the task is considered critical.
Click Set as Default if you want to make this setting the default for all your projects.