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A three-focal-point method is used for the design of a flat-top planar waveguide concave grating. With the Gaussian approximation a modified formula is proposed and is compared with the numerical model of the scalar diffraction theory by using a design example. The sources of loss caused by the flat-top design and the grating are taken into consideration. The optimal separation between the two outmost focal points is obtained and the spectral response of the demultiplexer with a ripple below 0.04 dB is achieved.