Napkins.

I’m pretty much a “back of the napkin” guy.

Back of the napkin calculation: You have 3 units. 

and enough pots pans and eating stuff to last 8 days, or would you need maid service more often.If the sink has a hand held squirt nozzle and a soap filled sponge wand it's easier than doing dishes by hand.The maid could change the sheets and towels, vacuum, cleanse the kitchen NSF sanitary (including the fridge), take the garbage to the curb, put down some carpet fresh, etc...Let the boys divvy up grass, snow shoveling and who makes the pot roast, carrots and potatoes for that night (that's a real muscle building meal).If you do go with a maid, it might be wise to pick one they won't be hitting on all the time, to ensure she is there to get her work done vs etc...

Investors must know that these rules (more accurately “guidelines”) which were intended to be a quick back of the napkin calculation to see if the investment was worthy of further review and nothing more.

see  https://www.biggerpockets.com/renewsblog/2009/10/0...So back of the napkin proforma rental revenue and cost guesstimates are crucial.  

For back of the napkin use rents times occupied pads with 40-50% expense ratio and a ten cap.

napkin math for pretty much all of this 30 to 100 year old stock is 40 to 50% of gross rent will be used for the care and feeding of the unit.. so out of the remaining 50% you pay your mortgage  and whats left over is your positive cash flow.. some years you will do better some years not..  

You could even use a back of envelope to or even a napkin to make an offer, however in a real world, you wouldn't do that. 

Obviously there is some skepticism (that I share) that this deal is a 20% cap, but would love to see the 'back-of-napkin' on what's going on here.