Or a good ole school “Cash T” CPA trick IRS Agents use to sniff out people who try to pass information that doesn’t meet the scrutiny of a basic truth income formula you can do in 2 minutes on a napkin.

In any event, if you cant make any money with a worst case back of the napkin calc (=50k) I would think twice before I'd proceed and spend a further dime on it. 

You can write it on a napkin and the title company's closing officer can help you through the rest.

Back of my, uh, cocktail napkin math says these units need to be renting for about $2400 per month to pencil at $280,000 per unit.Not asking you to 'give us the numbers' but at least tell us what the avg rent per unit is.  

There are other measures you can use but I have found these to be helpful for a quick “back of the napkin” analysis.

They will not accept things such as paper diapers, sanitary napkins,tampons, children’s toys, wads of toilet paper, balls of hair, grease, oil,table scraps, clothing, rags, sand, dirt, rocks, or newspapers.

When I do some napkin math about Milwaukee I figure we have about 8,000 leases signed every month.It's a little like the joke about running from a bear, you don't have to run faster than the bear, just have to run faster than your friend.

If its a back of the napkin estimate then I’d start with the highest significant digits and round up to get an order of magnitude.

, or is there a back-of-the-napkin way to get a rough idea to see if this approach is even viable?

A contract can be made on the back of a napkin if both people agree, but you'll want safeguards in place to protect your earnest money and liability so a state form, realtor form, or custom contract would be better.