This could be a good opportunity for the experienced PROs to may be off load some of their knowledge as well as some of their portfolio to the newbies, by signing a contract on a napkin :).

I've enjoyed backtracking on some of the podcasts, reading through Ultimate Beginner's Guide, and some of the most helpful blog posts that come to mind have been "Ultimate Guide to Analyzing Rental Properties," "Rental Property Numbers So Easy that You Can Calculate Them on a Napkin," among others.

I would like to simply write an I.O.U. on a napkin stating that I'll repay them +5% over the course of a year or so.How have you seen this done?

I guess it all depends on what your end goals are (Flip, Long Term Buy/Hold, Cash out Refi etc....This is just a quick back of a napkin # crunch but a cash out refinance may be a consideration especially if you want to hold onto it long term in your portfolio:  Purchase at $215k, 20% down payment = $172,000 initial loan balance.

Even on napkin sketches I would use quantity takeoff and base prices off of those quantities.A napkin sketch is where you only have a floor plan and no other information.  

So is a napkin and a handshake, or a verbal yes in NC (literally), it has held up in court before. 

Show me the back of your napkin.

And a third tenant I inherited, had such a bad cockroach problem, I had to evict her too due to sanitary conditions.

Great for a back of the napkin, quick analysis.

Do a back of the napkin FinanciaI analysts to determine if the numbers will work.