An important objective in the operation of low energy medical standing wave linear accelerators for medical X-ray use, is to limit the spread of energies into the low energy region in order to reduce skin dose to a minimum. This article suggests how this might be achieved by adjustment of the timing of the electron beam pulse relative to the rf power pulse. A theoretical model of a tube based on equivalent circuits is examined to support this proposal. In addition, parameters which affect the dose rate are optimized, in particular the beam loading, and the coupling constant between the accelerator tube and the rf power source.