2 definitions, return on investment and cash flow.

Cash flow is important as it is basic to hold the property for long time.

Return is more important as it is the target of investment.

Keep high leverage to no cash flow or a little positive cashflow. In long run, the return for investment is better.

this link may help you understand a 15 year return calculation.

http://www.duplexesoftexas.com/PDFs/RBS%2010.19.pdf