The estimation of the correctness of the calculation of the composition and properties of the final slag in the workstation of the blast furnace technologist is carried out. The developed model takes into account the complex effect of the components of the slag on its properties by calculating the viscosity polytherms in the range of available temperatures in the blast furnace. The results of statistical processing of more than 111 thousand issues, about 300 periods of operation of indicators of the slag mode of 8 blast furnaces of PJSC “MMK” are presented. The assessment was made on the basis of calculating the coefficients of pair correlation between the actual performance of individual outlets of the blast furnace and the indicators calculated using the process model. The pairwise correlation coefficient of the actual and calculated (according to the chemical composition of the loaded charge materials) content of individual components in the final slag was from 0.85 to 0.97. Comparison of the viscosity of slags of different compositions and at different temperatures showed that the experimental data corresponded to the calculated values with a relative error of no more than 5.4%.