Why calculate when you can guess? Here is what happened the other day.
I was working in a nice subdivision several miles outside of town when a couple guys with a trailer full of bark pulled up across the street. They ran short, and in front of the client as well - which isn't ever good. This meant having to go back into town for more bark. When you consider the embarrassment, cost of gasoline, lost time, and wages, it certainly makes sense trying to get your estimates as close as possible.