Your result might be reasonable to be different from simulation, as the formula you used is for uniform current density. However usually it is not. You will need to integrate it.
https://optics.ansys.com/hc/en-us/articles/360034398154-EH-charge-monitor-Simulation-object
You will need to interpolate the triangle mesh into rectilinear mesh.
I would suggest that you do not do this, as the current monitor gives you accurate result than you do it manually. Do you suspect the simulation result ?
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