Generally, to find the basis for depreciation:

1. Multiply the cost or other basis of the property by the percentage of business/investment use.
2. Subtract any section 179 expense deduction.
3. Subtract any special depreciation allowance (50% and 30%).

For more information, go to IRS Instructions for Form 4562 Depreciation and Amortization (Including Information on Listed Property) and IRS Publication 946 How to Depreciate Property.

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