An interferometric method using a nematic liquid crystal phase grating has recently been developed for determining the sum of the flexoelectric coefficients, (e(1)+e(3)) [C. Trabi, C. V. Brown, A. A. T. Smith and N. J. Mottram, Appl. Phys. Lett. 92, 223509 (2008)]. The switching response of the device can be simulated if the applied 0.2 Hz squarewave voltage in the nematic layer is modified so that it is enhanced by a reverse ion-field after each polarity reversal and then decays with time. This process has been investigated using measurements of the transient capacitance of a planar nematic layer to the same waveform.