[djmips]

It depends on your field of course. With experience, you will understand how to do things and you won't even realize how many mistakes you avoided by just not 'being stupid'. In my area it's often best to do a pre-production phase that is more of an R&D phase to explore the tech you need to incorporate or invent. This is where you iterate and yes optimize. It's hard to estimate at this stage. Then in the production phase you can have smoother sailing as the tech is established and the content is more or less production line. Estimates are accurate. Like a factory.