Since Kruger developed the theory for a single stage of a turbomolecular pump in 1960, it has been widely applied to the design and research of turbomolecular pumps for the past 30 years. One of the main ideas in his theory is that the maximum pressure ratio can be expressed as the ratio of the transmission probabilities of the molecules that pass through the blade row upstream to downstream to one of downstream to upstream, and that the maximum speed factor can be expressed as the difference between the above two transmission probabilities. Considering that Kruger's theory was derived under zero flow, we construct a modified expression for the maximum speed factor in this article. In addition, a second approximate expression for the case where the gas flow is between the maximum pressure ratio and maximum speed factor is proposed. The two new expressions are both verified using the direct simulation Monte Carlo method and show a good match.