In this paper, we expand a theoretical framework for modelling standing waves in millimeter/submillimetre optical systems. Standing waves or multiple reflections have been traditionally difficult to model but this analytical method proves to be very versatile in dealing with reflections between typical quasioptical components such as lenses, apertures and mirrors. Using a full multi moded description of the feed horn (typically a corrugate horn), which is then transformed to equivalent free space Gaussian modes, a full mode matching scattering matrix approach is utilized to analyze any optical path/element, in which track is kept of both the backward and forward propagating components of the fields. We present an analysis of the reflected power from complicated cascaded optical apertures in a situation similar to a stray light baffle. Interesting constructive and destructive interference conditions can be observed from these structures and can be uniquely modelled with the presented theoretical technique.