The detection of gravitational waves by the LIGO-Virgo-KAGRA network of multi-km laser interferometers has shown to be a powerful tool to probe astrophysical processes that may not be observable by other means. To overcome limitations that affect terrestrial interferometers, LISA (Laser interferometer space antenna) involves a triangular constellation where laser links will propagate powerful laser beams over 2.5 million km. Each of the three spacecraft contains two interferometer units to detect with a high accuracy and resolution the interference of local beams and the beams that have travelled over the long distance. Each interferometer unit is a complex set-up where multiple, cross-referenced, interference measurements are made, and in such a situation stray light comes in for a number of reasons (ghost beams, scatter at rough or contaminated optical surfaces, diffraction, imperfect polarisation) and can reach any of the photoreceivers, thus affecting the quality and stability of the measurements. We have implemented the "SL-OGSE" (Stray Light optical Ground Segment Equipment), an instrumentation inspired from OFDR (Optical Frequency-Domain reflectometry) and able, not only to measure the optical amplitudes for the different contributions to stray light, but also to retrieve the corresponding values of the optical path difference between the stray and nominal beam paths (https://hal.science/hal-04740519). The latter information is used as a hint to the determination of the actual path followed by stray light. However, in order to use this information with a good reliability, a complete and accurate modelling of the system under test has to be run so as to confirm the interpretation of each of the observed contributions to stray light. In this presentation we describe the techniques and the parameters used ro ray trace, with FRED software, stray light paths throughout a complex optical system. Simulations results are compared with measurements obtained with the SL-OGSE and show how simulations can foresee the most of the dominant stray light contributions. It is also highlighted in the presentation how the choice of the parameters to be used can influence the results, and how physical variations from the nominal in the setup can bring unexpected results, which require an additional effort to be fully clarified.