Cash Flow Analysis Vacancy Percentage for SFR?

[Clint Galliano]

I am wondering what is a good vacancy percentage to use in my cash flow analysis? Recommended values range from 5% - 40%. 

I want to be realistic, but I don't want too miss out on a solid performer because I was too conservative in my analysis. 

I currently have a SFR from an inheritance {my wife's childhood home) and while the CF is positive even with 40% vacancy in a practice analysis, it got me thinking to ask "What is a practical number to use?".