Resonant grating waveguide structures (GWS) are candidates for extreme narrow line reflection filters. In contrast to conventional dielectric mirror designs, a GWS may be composed from a single high refractive index film with a diffraction grating on top or bottom. In the high-refractive index film, a guided-mode resonance mechanism may theoretically lead to 100% reflection efficiency, while the system is only merely reflective off-resonance. The theoretical treatment of these systems is complicated because it combines optical interference coatings theory with the theory of diffraction gratings. Hence, typical thin film design programs are at stake here and must be replaced by grating solver software, which performs these calculations within the rigorous coupled wave approximation (RCWA), but is not always convenient for reverse search tasks, among them the system design. We derived approximation formulae that allow to analytically estimate film thickness and grating period necessary for a required peak reflection wavelength, assuming a vanishingly small grating profile depth. The estimation formulae work for normal as well as oblique incidence and both types of light polarization. In addition to all-dielectric GWS, we discussed the case of a GWS with metal components. In this case, instead of selective high reflectivity, the system woks as a selective high absorber. The derived formulae allow a straightforward and simple estimation of the film thickness for any resonant GWS. All results have been confirmed with RCWA-calculations and first experimental data. We discuss the merit of peak wavelength and width estimation, as well as the effects caused by absorption of the high refractive index film material.