A very informative characteristic of the scattering properties of a mirror is the PSD function describing statistical properties of the surface irregularities in the spatial frequency domain. We have analyzed applicability of the PSD-based evaluation of beamline performance of prospective (before fabrication) x-ray optics for XFELs, when dedicated optical systems deliver the highly collimated coherent x-ray beams over distances of hundreds of meters. We have shown that in this case, the classical specification based on the PSD of a surface error distribution, measured with a single existing mirror, is not applicable. In the XFEL case, the mirror surface errors at the longer spatial wavelengths, comparable with the mirror length, have the greatest effect on the quality of the reflected beam, leading to the appearance of speckles in the focal plane. In this respect, we have pointed out two fundamental problems. First, the stability of measured PSD spectra of existing mirrors at longer spatial wavelengths is unavoidably poor. This is associated with an unavoidably large error in the determination from a single measurement of the corresponding PSD function. Second, a truthful prediction of the scattering caused by long-scale surface irregularities appears to be impossible, even though the PSD is known exactly at all spatial frequencies, because the dispersion of the scattered flux was demonstrated to be of the order of the flux mean value. Therefore, the only way to predict the future mirror quality is the use of a probabilistic approach, i.e., determination of the mathematical expectation to fabricate a mirror that provides the peak-to-valley value of the radiation irregularities on the sample surface lower than the required one.