Cash Flow Estimates
Cash flow means different things to different investors.
Some investors like to think that positive cash flow is when the gross rent is greater than the mortgage payment.
For the sake of many of our calculations we use a little bit more complicated approach and say that cash flow is the difference between the rent collected (minus a vacancy allowance for times when the property is vacant) and the expenses of the property including (all loan payments, taxes, insurance, estimated maintenance, utilities and any other expenses like a monthly HOA fee).
Sometimes, we do use zero for some of these numbers.
For example, if the tenant normally pays the utilities I usually use zero for utilities. However, if you really wanted to be more accurate in your own estimates, you could estimate what you spend on the utilities you need to pay when the house is vacant and make that a monthly expense.
If you will be managing the property yourself, you may opt to use zero for your property management expense. I tend, although I do not always, put in a property management expense.