5 minute Look at a property question

[Brian Stark]

I hope this is in the right forum.  Apologizes if it's not. I'm looking at my first buy and hold rental.  I want to get a multi plex and I found one at random locally on loopnet.  I just want to make sure I'm looking at these numbers right for the 5 minute quick analysis talked about in Brandon's videos and blog posts.  Can somebody help double check me?

Property is listed at 240k

It is a 4plex that is fully rented out and all units are 2 by 1's and rented out for $500 a month (seller info provided not confirmed)

Using the 50% rule I would get approx. $1000 to pay for mortgage and cash flow in pocket. ($2000 in rent X 50% = $1000)  Wanting to cash flow positive for $100 a door that leaves me with about $600 to pay for a mortgage.  Playing with a mortgage calculator I would max out a conventional loan at about 110k to get the payments at $600 a month. And that's just the loan amount no counting anything down.

Since the numbers don't jive with out something drastic like a crazy low mortgage with a crazy high down payment or way low balling an offer, or something like raising the rents to $700+ to get a mortgage clearance of 1k a month payments.  This is an offer worth passing on because it will negatively cash flow or just barley break even at the end of the day.

Am I applying the rules for the 50% quick test right?