In this paper a new idea to help to predict the aspect ratio scaling law for the toroidal mode number of the so called Single Helicity states in reversed field pinch (RFP) plasmas is presented. In particular it is shown that the pure electromagnetic response of a toroidal shell is 'optimal' when currents with specific pitches are excited on it and that this response depends on the shell aspect ratio. The pure electromagnetic passive toroidal shell response model, cannot however, be useful in predicting the handedness of the current which is related, instead, to the plasma magnetic field profiles and their on-axis stability properties.