[jadams3]

The most useful piece of advice I have gotten for estimates is that they are all junk until someone sits down and tries to do the work. For big jobs, 20% of it, small jobs pushing 50% of the work.

In every single case when you've done that much work, I seem to wind up with a reasonable estimate.

If we've done the job before and have data on it, also reasonable.

Double or triple anything else.