@jhwgw
The tax values are used *only* to determine percentages. Nothing else. You stated the property tax bill indicates 20% of the tax value is for land and the remaining 80% is for the structure. Since your cost basis is $809,000, math indicates 20% of that is $161,800.
So in the cost box you'll enter $809,000 and in the cost of land box you'll enter $161,800. From there the program (not you) will do the math and assign a value of $647,200 to the structure, and depreciation will be based on that value for the structure. The actual depreciation you're allowed will be further reduced based on the percentage of floor space the home office occupies, date placed in service as a home office, and a few other things.
