If you have clean zero crossings and undistorted current, then the best method for your controller would be to calculate the lag/lead by measuring the time between zero crossings. and then using a look-up table. Or, if you can spend money on a energy meter chip then you can read the true power and the apparent power and then calculate the PF. Note that, the accuracy of the first method will depend on the current waveform and table length. As others rightly pointed out, whatever the energy meter does, can also be coded, but I doubt whether the controller mentioned can handle that load. A DSP would be much better.