C&N method is purely empirical ... you should use them once you have a very good step rate test (you should be carefull on the pressure you select to calculate the C and N as this method applied only on PSS state... try to calculate the stabilization time)
however the forcheheimer is more related to the Darcy equation where you have deltaP= AQ^2+bQ ... (B represent the Darcy flow and A represents the deviation from Darcy flow due to turbulence) .... so A and B (logically) could be obtained without having test data (if you have permeability and D factor)

One big difference between them is that C&N doesn t take into account the Pseudo pressure (in other words C &N do vary with time ... as the pressure is declining... mu and Z factor are changing) however the new feature in forcheheimer can be writen in m(p) ... which will make them fixed for the entire prediction .....


To make it easy ... Forchehimer Pseudo pressure is more accurate