X‐ray diffraction rocking curves of a strained‐layer superlattice structure have been measured at symmetric and asymmetric Bragg reflections and compared with simulated diffraction curves. The calculations are based on dynamical scattering theory. The experimental and theoretical curves exhibit a discrepancy with regard to the angular position of the higher‐order satellite reflections, which can be removed by introducing a new expression for the deviation parameter in the dynamical diffraction theory. The improved deviation parameter extends the range of validity of the two‐beam approximation, especially for asymmetric reflections with glancing exit geometry. Therefore, if the relaxation of strained‐layer heterostructures is to be determined by comparison with simulated rocking curves, only the improved parameter should be used.