Yea you are definitely right about the assumptions being made but the only assumption is in the "1.8" of the equation. There is very much so no perfect way to create an equation when it varies from person to person. The 1.8 is the closest estimate I could find that would, in most cases, create the answer that each person is looking for. In some cases it may vary up to 7 or 8 hours in either direction but I would say that that would be the extreme.

As for your 15 battles idea, yes that would be the ideal variable for each person to individualize the scenario. Yet I wanted to make the equation simple so as to be close to accurate, never perfect. When you find your answer I would leave room a couple hours in either direction for the assumptions made to fall into place.

*EDIT* and thanks Sarin xD