Something else I just caught in your original post.
percentage of business use (the fraction of days of occupancy that are rentals), and dividing by the depreciation period for residential rental buildings.
Totally wrong. For rental property, you use the MM (Mid Month) convention, regardless of what day of that month the property was placed in service.
See IRS Publication 946 at https://www.irs.gov/pub/irs-pdf/p946.pdf and use the MACRS worksheet that starts on page 36. For item #6 of that worksheet, use Table A-6 on page 71.
