In two-pole Induction Machines the shaft leads to a fictitious increase of the inner yoke in the rotor. This effect is influenced by axial cooling vents in the rotor. Pre-existing analytical calculation methods estimate the magnetic voltage drop in the rotor for only one path of the magnetic flux. This is done by simplified boundary conditions and leads to inaccurate results. In this paper a semi-analytical method is introduced. Therefore the branch current method is used and the solution of the nonlinear equation system is disclosed. The results of the analytical calculations are validated by Finite Element Analysis. The involvement of the semi-analytical calculation in the magnetic circuit is illustrated. Further refinement steps are described. Applications for the semi-analytical calculation method are introduced for machine design considerations.