I've tried using a stopwatch along with the editor, but I've had dissapointing and frustrating results.  Sometimes the ME tells me that a segment is going to run 30 seconds, but when I actually run the mission, the segment only runs 24 seconds.  Maybe time in the game is not paced the same as my stopwatch.  Or maybe I misunderstand what the ME telling me about my segments.  In any case, I think it might be helpful to see the mission time in the editor.