Am I understanding the 50% "rule" correctly?

[Nathan Remington]

Hi, I am looking to purchase my first multi-family property. From everything I read on posts and hear on the podcast the 50% rule says that you should save 50% of your rent to cover expenses and the other 50% should cover the mortgage PLUS give you $100+ in cash flow. Is this correct? Would this apply to me if I am using an FHA loan and only putting 3.5% down? My mortgage would be higher with the small down payment. Should I even expect cash flow with that downpayment or does this rule only apply to 20% down?