I don't think there is a clear-cut answer.
The Home Office form is deduct a proportionate amount of building (mortgage interest and depreciation), real estate taxes, utilities, repairs.
A "finished" area (regardless of the ceiling height) would definitely be used because that fully contributes to those deductible items.
Unfinished areas, such as a garage, PARTIALLY contribute to the expense of the building, taxes and utilities, but often at a much lower rate than a "finished" area. So one could argue either way if they should be used or not.
