Subfloor flatness is a serious thing here at J.L.Vivash. We are looking for subfloor flatness to a degree of 1/8” over 6 feet. This is not only an NWFA recommendation its also a warranty requirement in a direct glue down. We feel its just good practice in any lifetime floor. We use aluminum straight edges and lasers to check for flatness. To level on plywood subfloors we sand high spots and glue and staple various thicknesses of plywood to low spots then feather out with sanding. On concrete or gypsum subfloors we grind high spots with diamond grinders and use fast curing cementuous self levelling products to build up low areas.