How much money should you set aside per door / single family property for emergencies such as covering the mortgage due to vacancy? That's my primary concern with brrrr or any property I don't own outright. 20% or less down is great but if my tenant leaves I have to cover the mortgage which could be 2K a month in Northern Virginia. 

Depends on the age of the property and what your local vacancy rate is.  How old is your water heater, ac, etc?  [Cost to replace everything / years of service + expected yearly turnover costs) ] / 12 = monthly set aside into a high apr savings (capex).  

Howdy @Ian McKay

Those are the numbers that you are including in your Cash Flow analysis. I would use 10% for Vacancy until you can verify the actual rate for that area. Also use 10% for CapEx until you can have the property inspected and determine what the condition and life expectancy of all major components and appliances are. Use the report to determine what you need to repair/replace immediately and what deferred CapEx reserves requirement will need to be. Use the calculations that @Michell P. provided.

Thanks! 

Well this was all good and quick info! Thanks Everyone!