FDTD can give you accurate simulated H fields. If you simulation does not take too long time, it will be much better to resimualte it by enabling the recording of H.
In case you want to calculate H from E, things can be different. This is because, FDTD is discrete method, and E/H components are displaced:
the small space dislocaiton can cause power calculation incorrect depending on settings.
To avoid this, the regular output of the frequency-domain monitor is already interpolated into one point for each Yee cell. However, the plane monitor does not have z span since it is in xy plane. So you will need to use farfieldexact to project the fields to dz. Then you can calculate H. However, dependig on the gradient of fields, the result might be depedent on dz.
Another method is to extrac the raw data using option 1: https://optics.ansys.com/hc/en-us/articles/360034409834-getdata-Script-command
Please refer to the script in this example: https://optics.ansys.com/hc/en-us/articles/360042165534-Planar-silicon-solar-cell
Please note the FDTD mesh is usually non-uniform. For better result, please interpolate to uniform mesh when you calculate the difference.