Like I said, I am just starting the research - call this the napkin.

And, I have an ongoing issue with the tennants flushing improper items down the toilet (tampons, sanitary napkins, baby wipes ect.).

With a husband in tech I'm sure one of you has already done the quick back-of-the-napkin math.  

Back of the napkin calculation  let's just say these would produce $5,000 a month gross and $3,800 NOI, loan payments ~$1,300 leaving $2,500 cash flow to the IRA monthly.  

I found this document related to PA:http://tenant.net/Other_Areas/Penn/harris/pa-part2...Two paragraphs appear to relate to this situation:Under the new "implied warranty of habitability" law, the tenant's obligation to pay rent and the landlord's obligation to maintain habitable (safe, sanitary and fit) premises depend upon each other.

It has been taught for "quick analysis" or "back of the napkin analysis" by many different real estate investors for decades, long before you started investing.

Use all electrical, plumbing, sanitary, heating, ventilating, air conditioning, and other facilities and appliances in a safe and reasonable way.D.

Is there some rule of thumb/back-of-napkin easy way to guesstimate typical the difference?

Not sure of your definition of the stock market so will stop there.2)  back of napkin calculation  Option A take current balance of your 401k and make assumption on your compound annual return until 59.5. 

Bill Robinson,You certainly don't need me to "run the numbers" or suggest the easy stuff, but my back of the napkin math putsyour current combined PI at about $575/month.