We discuss the possibility of applying the semi-empirical formula derived for evaluating the separative power of the Russian-type optimized gas centrifuge, with a rotor length about 1m, to centrifugal machines of arbitrary length. It is demonstrated that the formula can adequately describe the dependence of a single gas centrifuge's performance on rotor peripheral rotation, length, and diameter for machines with lengths of up to 5m. The comparison of the calculated values for separative power, obtained by numerical simulations of flow and diffusion in gas centrifuges with that of the URENCO machines, demonstrates satisfactory agreement.