Calculation of the geometric part G of the profile shape is part of the recently developed fundamental parameters profile model. A Monte Carlo raytracing algorithm is used to describe realistic conditions such as multiple slits/collimators, dimension of secondary monochromator and complicated specimen shape. To model the X-ray source more exactly, we successfully introduced a correction for Xray tube tails. A relevant part (6%...30%) of the X-rays is produced not from a proper rectangular line focus shape but from supplementary tails on both sides of the optical focus. Thereby, the measured profile shapes differ from those simply derived from a box-shaped focus. Applying an X-ray tube tails correction allows the use of fundamental parameter description for line profile (micro strain) analysis.