Yes, the filter is to smooth the monitor data and make sure the intensity at the monitor edges is small enough. Thus if the monitor (and simulation region) is large enough you do not need to use it.
For particle on substrate, you can try to use 5 box monitors in analysis group, or use large FDTD and monitor spans to pick up all the scattering.
https://optics.ansys.com/hc/en-us/articles/4403945613843-Polar-plot-of-scattered-field-with-particle-on-substrate
If box monitors are in lossless material, the distance does not matter.
You are navigating away from the AIS Discovery experience