You’re confusing the numbers somewhat.
Let’s start here… Gross rent is $2400 per month.
50% of $2400 is $1200.
So that means $1200 per month goes toward operating expenses. In this part, we include everything but your principal and interest on the mortgage and your cash flow. Operating expenses would include insurance, property taxes, vacancy allowance, maintenance, repairs, legal fees from evictions, fees to do your taxes each year, allowance for income tax, any utilities you would pay as the LL, utility connection fees to get them turned on for inspections, your printer paper and ink to print leases / lead based paint disclosure forms and pamphlets, etc.
The other $1200 goes toward your mortgage and anything left over is cash flow.
So in your scenario, the $456 mortgage would be subtracted from $1200 and leave you with $744 cash flow per month for the building.
The other $1200 would have to cover all of those other expenses. Of course, we don’t actually know how much maintenance, repairs, tenant damage, etc will cost over a period of time so we use the 50% rule to estimate and make allowances for that. When you’re analyzing deals, you can nail down certain costs like property taxes, insurance, and to a certain extent utilities too. The more sure you can be of those numbers will help you determine if what is left over will be enough to cover the expenses you won’t know (when will plumbing go bad, how long will that wood deck last before it needs replaced, do I have 10 years left on the roof or closer to 5 years, etc).
So let’s look at your example again.
You have $1200/mo for operating expenses. You subtract $337 for taxes and $200 for insurance. This leaves $663 per month for all other expenses. On months when you have one vacant unit, that’s a loss of $600 in rent so that only leaves you $63 that month before it cuts into your cash flow.
Some months you will have much higher expenses than others due to things like repairs. What we’re trying to do here is estimate the deal overall while leaving a sufficient amount for the what if’s and determine if the deal makes sense to make you money over time or if you’ll be cash flow negative from it.
Something else to consider on these smaller multi-unit buildings is which utilities you have to pay as the LL. Many smaller buildings don’t have the water separately metered for each unit. Some have electric/gas split while others do not. We own a six unit building where we pay $31.90 per month for a trash dumpter. We also pay electricity on the hall lights and electric for the water heaters. We also pay the water bill. On the duplex we own as well as another property that has a 2br house and a 1br bungalow, we pay the water bill (which includes trash service too).
Water isn’t too bad to pay for as a rule (unless a tenant has a water leak and neglects to tell you), but paying for someone’s heat can get very costly. Just some other stuff to consider with these investments.