Work in vacancies, personal property tax, and the cost of CapEx items into that equation. 

Most newer investors start with the Realtor piece of the equation.  

If you give address or location I can get a lot more accurate:In 2015, there were 269 fourplexes sold in Maricopa county for a median sales price of $182k.So far in 2018, there have been 136 sold fourplexes in Maricopa county for a median sales price of $295k.Using that figure, the yearly appreciation rate for fourplexes as a whole in Maricopa county has been 17%.You'd probably want to get the figure for the last year to really narrow it down, that way you take any bounce back from the recession out of the equation completely. 

It does not account for the other side of the total cost equation...housing prices. 

BRRRR might help you recycle cash and be more effective long-term, but you have to add far more hustle into the equation to make it happen. 

How does NOI enter into the equation?

You judge it by a metric that take debt service out of the equation.

It's pretty common practice to use a HELOC drawn from the equity in one's home to serve as the down payment for investment real estate... yet I haven't found any calculators that will allow me to add that into the equation when it comes to deal analysis.

@Jared Newsom I would normally agree with you but the vacation rental is the wild card in this equation.

It makes you take all emotions out of the equation and makes it just a business.