You are optimistic anyway, so you tend to be aggressive. But then you say "Ok, I'm aggressive, so, therefore, I want to try to be conservative." So you make some estimates and then, maybe, you double it.

Well, you're really probably wrong by 6x. What I've always found is that things are 3x as long and cost 3x as much as I tend to think when I go into it. So you have to prepare for that.