Recently introduced generalized Snell’s law provides wide possibilities for wavefront manipulations using metasurfaces. In contrast to conventional blazed gratings, their metasurface-based counterparts are planar (no grooves) and do not require complicated fabrication techniques. However, all previously demonstrated metasurfaces for anomalous reflection have revealed their deficiency due to parasitic energy coupling into non-desired diffraction modes. This negative effect becomes especially pronounced for metasurfaces designed to have a large separation angle between the incident and reflected beams. The reason is the used inaccurate approach for metasurface synthesis. It approximates that the surface is uniform on the sub-wavelength scale and the phase of the local reflection coefficient follows the linear profile dictated by the generalized reflection law. While the former assumption could be made, the latter one, as we show in the presentation, is incorrect. Moreover, the conventional synthesis approach does not take into account requirements on the amplitude of the local reflection coefficient. In the presentation, we will demonstrate an original alternative design scheme for metasurface gratings based on the generalized surface impedance model. It appears that perfect coupling of an incident plane wave into a single reflected plane wave requires energy channeling along the metasurface plane. To verify our design approach, we fabricate and measure an optical metasurface that reflects a normally incident beam at a very steep angle of 80 deg. The comparison of the two approaches shows that our new scheme provides double increase in the efficiency and complete absence of parasitic reflections.