Be aware, many helmet mounted rear lights are not visible when you are leant down on the bars, especailly combined front/rear where you might have it mounted further forward so it points forward correctly.
I saw 3 people (other than me) with rear helmet lights this morning on my commute, none of the rear lights were visible while they were riding, only when stopped at the lights and sat up away from the bars. Im sure they thought they were visible all the time.
[EDIT] also worth noting i was on a bike (and 6ft4) so was also much higher than the eyeline of drivers and still couldnt see the lights.