My experience with the condos I've used with AirBNB is excellent.  

Excellent credit and looking at put 20% down.

I guess it depends on how many you have and how much income the ones you have are generating.I really think you need to look at reserves as a sliding scale.If you own between 1 to 5 - I think you need about 5k per house.Between 6 to 10 - About 3,500 per house (i.e. 20k for 6 houses is good, 35k for 10 houses is fair).Between 11 to 15 - 3k per houseAnd 16 to 20 - 40k in the bank should be sufficient.20 or more - 50k should do it.That also assumes that you should net about 200 to 300 per house or more on average.So technically, if you have 20 houses, you should be making about 4 to 6k net profit (after vacancies and basic repairs).

Or if you really want to geek out I have a great excel calculator that will give you all the info you would need plus principal / interest for every payment over the life of the loan.  

I'd like to slide in on the spreadsheet if possible

Ryder,@Jeff Copeland manages my 14 units and does an excellent job! 

Here's another article you may find useful if you insist on using excel and are interested in the IRR (Internal Rate of Return). http://www.privatemoneylendingguide.com/investors/articles/hard-money-loans-irr-internal-rate-of-return

I've had excellent return on investment from these "little ATMs" as I call them. 

There are so many sales pitchy products that are going to help you “take your business to the next level” or “double your income” these people dont make money by me being successful, This is an excellent point.!!! 

However I am guessing you will need excellent credit and low dti.