An engineering technique is proposed for assessing rational ranges of variation of design and operating parameters in a three-stage method of mixing grainy materials. The technique takes into account the standard value of the heterogeneity coefficient of the resulting mixture and the physical and mechanical properties of the working materials. The calculation is based on stochastic models of mixing solid dispersed components for two stages (steps) of performing this technological operation during the operation of the apparatus at each of the three stages. A distinctive feature of the proposed engineering technique is the use of not only the modified Froude criterion within the framework of the Pi-Buckingham theorem, but also analytical representations for the distribution functions of the number of particles of the mixed components by the characteristic parameter of the mixing process. At step 1, the angle of scattering of the said particles by the brush elements after leaving the drum-tray gap is selected as such a parameter, at step 2 - the angle of reflection of particles of rarefied flows from the baffle surface. An example of calculation is performed. The influence of many design and mode parameters of mixing from the obtained rational limits of their change on the process efficiency indices is studied. A decrease in the ratio of the extreme values for the angles of dispersion of particles of the components being mixed (step 1) to the index of 1.05 is observed.