How do IRS determine how much depreciation recapture I need to pay?

I bought a property in 2012 and start to use it as a rental property in 2016. But I did not claim any depreciation in my last 3 year's tax return since my rental income is almost 0 without the depreciation anyway. But I just learn that IRS will charge depreciation recapture tax no matter if I take depreciation or not when I decide to sell the property. 

My question is if I start to do depreciation this year, how should I determine my Cost of asset?

Should I use

1) the purchase value I paid in 2012 minus the land value of my 2016's property tax bill?

2) the purchase value I paid in 2012 minus the land value of my 2012's property tax bill?

3) certain % (say 70%) of the purchase value I paid in 2012?

 

Will IRS always use the same cost basis I reported in my tax return when they calculate depreciation recapture?  Or they have their own way of doing that? If so, what kind of cost basis they will use? I want to make sure I can use all the depreciation allowable in my tax return. 

thanks