50% Rule - how much Cashflow per door after?

@Samuel Carmichael besides what @Brandon Ingegneri said about looking at absolute dollar amounts to figure out if it's even worth the aggravation, I also agree with @Michinori Kaneko about looking at percentages.

Personally I look at percentages first, namely 1) cash on cash return (yearly money back after all expenses including mortgage, divided by all money invested to get it rentable) and 2) debt service coverage ratio (net income after operating expenses but before mortgage payment, divided by the mortgage payment).

The first is (obviously) a measure of return, while the second is more of a measure of risk as it tells you how much of a buffer you have between the property's net income and the monthly fixed mortgage payment.

After you get a little more experience you'll also start to factor things like replacement reserves into account. You might want more money per door on a property that has more deferred maintenance simply because you'll know you're gonna have some bigger expenses coming up on the property sooner rather than later.

@Brandon Ingegneri