When it comes to real estate there are rules to follow if you want to rent out properties. 50% rule and general rule of $200 cash flow+ a month. Now where most investors get wrong is rent-PMI = cash flow. That is not the case.
Take the mortgage for instance is $500 a month and MARKET RENT is 1300. You take 50% of market rent which is $600 and that 50% is TAXES, INSURANCE, REPAIRS, VACANCY, MARKETING. 50% is a proven system where many believe its too much but it is averages over time
$500 + $600 = 1100, leaving with a monthly $200 cash flow. The hardest part is finding deals good enough, saving for a down payment, and determining true market rent.
There are other systems like 10% cap rate, 2% rule. But I think the 50% is the easiest to follow. http://www.123flip.com/education/the-50-rule