Maybe I'm just terrible at mathematics, but I'm struggling to figure out how to calculate ROI from a house purchased via my HELOC.The numbers I'm giving below are just examples.Let's say that I find a rental property which costs $50,000.

Don't think because they are "big" guys that they are right.I started this way as well and I learned early on that most people here have no clue what they are talking about.So back to our mathematical prove and disprove theories lesson...So take my case...and to add..

It may not be sexy but the best mathematical decision may be to pay off the high interest debt. 

@William WironenOnce you cash out from the refinance, you’ll have 0 cash left in the deal, so mathematically your return is infinite!

Foreclosing ASAP is a way to get in front of that mathematical "certainty" and cut my losses.By contrast if 4% is a normal interest rate and 8% is "normal" annual appreciation, equity (ie, the safety of my position) is actually still going up each year.

It depends on what you want to do there are many people on BP who advocate what I consider to be a house of cards with borrowing everything you can to maximize mathematical return. 

i use erentallock.com on mine. they dont require wifi, or internet. they use mathematic algorithms to spit out a code based on the time of day and length of stay. theyre typically used for VRBO but i use them on my rentals. codes are good for 2 years max.

It is a mathematical equation to get you to a financial goal.

Fourth, the cap rate is mathematically factored into the subject's calculated NOI and an opinion of value is developed based on that formula. 

Burst pipes, vacant units and leaky roofs can be accounted for mathematically, and this business is old enough that we do not have to re-create the wheel!