I will be writing my exam in a couple of weeks and I am getting confused with how to calculate Late start and Late Finish.  Some people calculate late start using late finish minus duration plus one (http://www.youtube.com/watch?v=Cai3PllvbTY), others just do late finish minus duration (http://www.youtube.com/watch?v=tT9KbnvRrTE).  Which was is correct?  Please help!