Ok, now it gets interesting. My dictionary lists dead reckoning, but acknowledges DED reckoning. There seem to be naval references to Dead Reckoning dating to the 1600's, although there is no explanation of the source for Dead. It acknowledges that DED reckoning has stronger rationale.
I am sure I have read something by someone serious saying it should be DED reckoning. This may have been something like one of Max Conrad's books, or something of that stature.
So, I now have no idea. So for the moment you say potato..........