Hi Chee,
As I mentioned before (and it was indicated by Damian in the previous message), to calculate the spectral efficiency of a certain channel (for telecommunication system which uses modulation, i.e. not baseband) you need to consider the channel spectrum of your system. If in your case you are using WDM system with the determined frequency grid, you can refer to this grid to find your bandwidth (see Damian message). You can measure this spectrum (for your task) in a channel after your performing modulation.