In the Bragg construction both the incident beam an the scattered beam are inclined to the (hkl) plane at an angle q making the angles of  incidence and "reflection" equal.  This can be achieved in the Laue equations by making the angles which the S and S₀ vectors make to the reciprocal lattice vector equal. Call this angle n.  Remember that the reciprocal lattice direction is perpendicular to the (hkl) plane.  These conditions can be shown on an Ewald sphere diagram.