In the presence of Deadlines, Constraints, variable Calendars, and resource leveling, Total Slack becomes unreliable as an indicator of the Critical Path (or of nearness to the Critical Path). For projects where the project completion is designated by the last task in the schedule, I use the Near Longest Path Filter to keep an eye on next week’s concerns….
See also a related blog entry: Tracing Near Longest Paths with BPC Logic Filter