The Photometer and Photometric grid in Spotlight use a simple cosine distribution falloff based on the peak candella of the lighting device with a given distance, beam and field angle. These calculations do not take into account any geometry that may be blocking the light, reflections, or ambient light.
If this is going to give you an accurate enough calculation, you simply need to fill out the candlepower, beam angle and field angle fields in the light info record and attach the record to your symbol. Then follow the procedures outlined in the manual for placing and focusing lighting devices.