I also find this difficult to understand. Are these distances in an XY plane? Given in columns?
Perhaps you are trying to use a reference measurement which is made from a calibrated source at some y position with x = 0?
If that is correct: Are you trying to correct for distance by inverse square law? Are you correcting for the cosine factor in the angle of incidence? And are the sources Lambertian emitters?
We may need a picture.
