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Non-incidence flow rate determination is necessary in the calculation of gas-dynamic characteristics of a centrifugal compressor by the Universal Modeling Method, since the magnitude of incidence losses depends on this. The direction of the critical streamline is affected by the decrease in the flow area by the blades of finite thickness and the load of the blades. In the course of primary design, the Universal Modeling Method uses a scheme of replacing the influence of the blade load with the effect of a vortex with identical circulation. Finally, the value of the inlet blade angle is selected by calculating the inviscid flow around the blades. For impellers with small design flow coefficients, the condition of the non-incidence inlet for the primary design and for the calculation of the inviscid flow is significantly different. The correctness of the calculation of the non-incidence regime for non-viscous flow was checked earlier by measurements of the flow in the impellers. CFD calculations and no-viscid calculations of twenty impellers were made in a tenfold range of design flow coefficients. The identity of inlet conditions by both methods was shown. To increase the accuracy of the primary design, the calculation model was refined. An empirical coefficient is introduced into the formula for calculating the velocity induced by the vortex. The analysis of data for 32 impellers with different blade profiling allowed to propose out formulae for calculating empirical coefficient, depending on the type of impeller, the blade load and the width of the throat at an impeller inlet. The calculation based on the new scheme with the empirical coefficient is accurate enough for the primary design. [ABSTRACT FROM AUTHOR]