I see that you have another post on this a while back that no one else has replied yet. Sorry about that.
I haven't used "scat_ff" a lot myself. But the page on Far-field projections from a box monitor mentions this:
Note: Particle scattering with a substrate
The 'box of monitor' far field projection is frequently used when studying particle scattering. This is fine, but it's important to remember the requirement that everything beyond the monitor box must be a single homogeneous material. When a substrate is present, this approach is not valid. Instead, the best way to calculate the far field scattering pattern is to use a single monitor, located above or below the particle (depending if you want scattering in the forward or backwards direction). You can then use the standard farfield3d function. When using a single monitor, it's important to make the simulation span large enough that most of the scattered light will pass through the monitor before hitting the PML absorbing boundary conditions.
Let me know if this doesn't help. I will get someone else to provide more insights.