The answer to "optimal capital structure" will change across time with the cost of capital. For example, it is better to have more debt when interest rates are low. For a bird's eye view on the topic, try Michael Milken's article, "Why Capital Structure Matters" in the Wall Street Journal (Opinion Section), April 20, 2009. You may be able to find an electronic copy of my book "Beyond Junk Bonds" (Oxford University Press 2003, Chinese edition 2013) in the university library -- chapter 8, of the same title, gives a more academic view on the subject. Good luck with your research!
This is such a difficult topic and there is unfortunately not one answer. Like Susanne mentioned, it is time related. However, there is definitely a limit to the extend of debt that should be used as an investor. You might want to read "Leverage, please use responsibly" for some ideas on optimal level of debt. Good luck.
For australia market residential property market, you can find out the breakeven point by using the weighted average interest, rental expenses incurred and the weighted average rental income.