Rental Propery Depreciation expense!!

Hello,

It looks I missed to take depreciation on my rental property in FY 2017 tax. Can you pls suggest, If I may need to fill amendment OR I may adjust this to next year (2018 tax).  Here is the scenario on my rental property:

The tax assessor's estimate of building value estimate is $392,700. My depreciation expense that you take each year against rental income would be $392,700 divided by the IRS allowed 27.5 years of useful life (residential real estate) for a depreciation expense each year of $14280.00

In Year 2017 Tax year. This property was rented out for 6 months (half year). So for Year 2017, I may take depreciation expense as $14280.00 / 2 = $7140.00.

Please confirm if this calculation is correct?  and I may take this expense deduction next year in 2018?

Thanks in advance!! 

Ravindra Kumar