Please mind that the assignment of the Landau levels critically depends on the Hall angle Rho_xy/Rho_xx. If it is much larger than 1 then your Rho_xx becomes promotional to the longitudinal conductivity Sigma_xx. As a result the peaks of Rho_xx oscillations correspond to the Landau levels. On the other hand, if the Hall angle is less than one then Rho_xx becomes promotional to 1/Sigma_xx and thus the valleys correspond to the Landau levels. If this step is properly done, the rest is simple you just create two columns one for the Landau indices (with whatever starting value) and the second columns is 1/B values. The last step is to apply a proper shift so that the interception between the line connecting the data points with the Landu level axis happens between 0 and -1.

Thank you very much for your answer.