Absorption Cross Section using Total Field Scattered Field Formulation

Does anyone know how to compute the absorption cross section for a lossy material? I have already determined how to collect the scattered field by placing monitors outside of the Total Field Scattered Field (TFSF) Boundary. I know that the absorption is J(w) / E_inc(w), but where should I place my monitor in regards to the TFSF boundary?

Should I compute it over the whole material or can I simply place a planar monitor on the opposite side of the plane wave injection?

Any suggestions would be greatly appreciated!