Bought out my ex-girlfriend - what's my new cost basis?

[Account Closed]

Years ago, I purchased my first primary residence with my (then) girlfriend. We each paid 50% of the down payment, each paid 50% of the mortgage and any improvements. We split everything equally. A few years later we split up, she wanted out, I wanted to "buy" her share of the property. I refinanced the mortgage in my name only, she signed quitclaim deed, I paid her what she and I agreed was fair for her share.

Fast forward a few years, the property is now a rental and I need to calculate my adjusted cost basis.

My intuition is that my adjusted cost basis is 50% of our adjusted cost basis when we were joint tenants, plus what I paid her, plus 100% of eligible improvements since then. 

Do I add what I paid her? She wasn't paid in escrow. Does that matter? What record keeping is required if I wrote her a check or or paid off her credit cards and/or student loan? 

Does the IRS base the answer to this question on what we intended to do, or what we did? 

Are there additional details needed to answer this question?

This is one of a number of RE tax questions I have. I'll ask them in separate posts so others can learn from my mistakes...:)

Thanks in advance for any input.