An in situ spectral calibration method for Thomson scattering systems, which uses a double-pass scattering system, has been developed for burning plasma experiments, such as ITER and DEMO. In such machines, space for diagnostics is very limited. For Thomson scattering diagnostics, laser injection equipment and collection optics should be installed in the same port because having multiple ports for a laser and collection optics requires a large amount of space. The design makes the scattering angle wide because the collection optics must be located near the laser entrance. In addition, measurements of high electron temperature (40 keV) are necessary. This paper presents an optimization of wavelength channels for the calibration method, in which two different spectra in the double-pass scattering are considered. Using the optimized wavelength channels achieves good accuracy in electron temperature and relative transmissivities even at wide scattering angles and high Te.