I do not know what caused your headache, whether dehydration or detox, but I think it is always better ot err on the side of caution.
Each dry fast I have done has gotten easier. I began witha 36 hour dry fast, then 48 hours, then 90 hours, then 5 days, then 8 days, and finally I did a 10 day dry fast. The interesting thing is that the final 10 hours of the 90 hour fast was just as difficult as the final 10 hours of the 10 day fast. I felt equally dry both times. So, I think as the body goes through the acidotic crises that Dr. Filonov talks about and cleans out toxins, a person is able to dry fast for longer without feeling as dry until much later in the game.