Nice share from our website member Engineer Mudassir Jamal about PDM Method. Mainly we have Forward Pass Calculate Early Dates hence the Free Float & Total Duration of the Project As longest Path, and Backword Pass Calculates the Late Dates hence the Total Floats & Critical Path of the Project.
FF (Free Float) = Simple formula to calculate Free Float is ES (of successor) – EF of current activity
TF (Total Float) = Simple formula to calculate total float is our usual formula i.e. LS (Late Start) – ES (Early Start) or LF (Late, Finish) – EF (Early Finish)