I think I can help you with two major points. The way I recommend clients to look at cash-flow property is discovering two numbers.
Calculate your Deal
The first calculation is the profit and loss for the property without finance payments. This will include everything from vacancy rate (25% is lender standard), maintenance, home warranty, insurance, marketing and taxes.
This will give you the actual cash-flow of that property. For example (quick and dirty example), you acquire a three unit property, each with rents of $1,000 a month. That would give you a monthly revenue of $3,000 per month or $36,000 per year.
Subtract expenses, for an example $500 per month. That means $2,500 in cash is produced each month.
Find your Break-Even point
Now you know that this particular three bedroom property. Let's say the value of the property is $500k. If you're a fellow BPer you probably already knew how to get to this point, then you go over to a mortgage calculator and see how much the monthly payment will be. This is fine, for a quick estimate but your next step should be calling a great mortgage broker. Let him know the numbers and tell you some financing options that will leave you cash-flow positive.
He may say it's not possible on some deals, others you might put have to use more cash, but more times than not they will be able to put you in a good position for positive cash-flow each month. That's the main goal, a check in your hand month after month.