About slack calculation
The amount of slack in your schedule indicates how long tasks can be delayed before other tasks or the project finish date are affected. If you know where slack exists in your schedule, you can move tasks when certain phases of the schedule have no slack and other phases have too much.
Total slack is the amount of time a task can be delayed before the project finish date is delayed. Total slack can be positive or negative. If total slack is a positive number, it indicates the amount of time that the task can be delayed without delaying the project finish date. If total slack is a negative number, it indicates the amount of time that must be saved so that the project finish date is not delayed. Negative slack indicates that there is not enough time scheduled for the task and is usually caused by constraint dates.
By default and by definition, a task with 0 slack is considered a critical task. If a critical task is delayed, the project finish date is also delayed.
When to use the Start Slack and Finish Slack fields
The Start Slack field contains the duration representing the difference between the early start and late start dates. The Finish Slack field contains the duration representing the difference between the early finish and late finish dates. The smaller of the start slack and finish slack amounts determines the amount of free slack available, that is, the amount of time a task can be delayed without affecting the start date of a successor task.