Apart from an experimental determination of the relevant correction factors or a full Monte Carlo simulation, the most popular computational approach to the problem of true coincidence summing effects is the point-source approximation. An improvement of this method is based on the so-called linear-to-square or "Third curve" of gamma-ray spectrometry. Both of these computational methods carry with them an intrinsic error, the magnitude of which we investigate for the case of environmental measurements with p-type HPGe detectors.