I get that, but that's the whole point, the world is not perfect and your first plan is usually gonna fail, for that you hedge and have plan b,c,d etc.. if i think im going to need 6 hours, I say 10 hours, maybe im done at hour 8, and im able to shock my client to the upside. opposed to forcing him to accept my poor planning and mediocrity as a de facto state of affairs.