This paper presents a new high accuracy window design method based on semi-infinite linear programming by using the Dual Nestled Complex Approximation (DNCA) algorithm. The paper presents a general window design specification, the formulation of the corresponding semi-infinite linear programming solution and several highly optimized window design examples. The paper illustrates the capability to enhance the sidelobe attenuation of the new upcoming ISO 18431-2 flattop window standard from the international organization for standardization. The procedure of enhancing the ISO flattop window can be directly applicable to any other existing windows. Flattop windows are commonly available in frequency analyzers for accurate amplitude measurements of harmonic tones in presence of noise. Moreover, it is possible to design high accuracy windows with more than 150dB sidelobe attenuation with the proposed method as well. Such high sidelobe attenuation is for example needed in today's 24bit equivalent 1bit-Sigma-Delta ADC:s, where high frequency noise is generated.