For the S=nz2 resonances in the ion cyclotron heating of Tokamak plasmas a simple formula is derived which relates the converted fast wave power flux to the local dielectric tensor elements in xx, in xy and to the fast wave electric field at the resonance. Here S= in xx, and nz denotes the parallel refractive index of the radiation along the background magnetic field. The validity of this estimate is analyzed for finite electron inertia and finite temperature and its usefulness in predicting the conversion at the edge plasma resonances is shown.