Sean, I don't go quite that detailed. Once you have done a few you will find your rythm but we break down our costs like this:
Outside
HVAC:
Roof:
Painting:
Landscape:
Windows: (don't do these much)
Miscellaneous:
Inside
Painting:
Floor Coverings:
Plumbing:
Electrical
Kitchen:
Baths:
Fixtures and faucets:
Micellaneous:
I may have left out a category, but that seems like all of them. If you break down rehabs into those categories and leave 20% for miscellaneous, you are usually within 15% of your estimates. Good enough. You will be wasting a lot of time and effort to get much more specific than that on the front end.
Now what we do to contain costs on the labor side is to walk the property and write up a scope of work. For a standard 15k rehab its about 2 pages and is detailed enough that the contractor should be able to understand 90% of what you want without asking. The rest just requires you being available when questions come up.
That might not be the best system in the world but its worked on a couple hundred rehabs for us.
