Setting Q for attenuation in PML elements/non-PML stabilization layer

[etotten93]

I am currently trying to add attenuation to my SPECFEM2D sims and I'm a bit
confused about how to set Q in the PML layers (I create these myself before running SPECFEM
and don't use cubit or other third party tools to make the mesh).

If you take a look at figure
3.4 in the manual there is a safety layer between the main model
and the PMLs. My questions related to this are:

1. Is Q set to 0 in both the PML elements AND the safety layer, or just the safety layer
   (where Q would be Qkappa and Qmu)?

2. Can the safety layer just be the edge elements of my model (bottom, left, right)
   with Q set to zero in those elements, or do I need to pad the model with these edge elements (width 1)
   in addition to the n_pml elements

e.g if we were looking at the model width, would the corrected model width become

x_width + 2*n_pml +2

or would it be

x_width + 2*n_pml, with the bottom, left, and right-most boundary elements of the model next to the
pml elements set with Q = 0?

Hopefully my questions make sense - it's a bit tricky to infer from the manual.
Currently, my code crashes when 'preparing attenuation within PML region'.

Many thanks in advance for the help @danielpeter @williameaton!