Adjusted Cost Basis Calculation:

Cost basis X business use percentage = Business portion cost basis

Business portion cost basis - Section 179 deduction = Adjusted cost basis

Adjusted cost basis X 30% Special bonus depreciation = Special bonus depreciation amount

Adjusted cost basis - 30% bonus depreciation amount = Adjusted cost basis to use for current and future year depreciation calculations

Example:
$21,814 cost basis * 90% business use = $19,633 business portion cost basis
$19,633 - Section 179 Deduction $10,000 = $9,633 Adjusted cost basis
$9,633 * 30% bonus special depreciation allowance = $2,890 bonus depreciation amount
$9,633 - $2,890 = $6,743 new cost basis for depreciation calculations for current year and future years