I forget the precise legal term for this, but basically the judge looks over your contract, examines what has actually happened, and then can overturn your deal using the "if it walks like a duck and quacks like a duck...it's a duck" principle. 

While interest payments Are deductible, you get back maybe $0.20 for every $1.00 paid in tax savings, every $1.00 still costs you a net $.80.So are you saying than that for every pretax dollar I pay down the HELOC principle with I can expect to pay about 20% of that total number in taxes?  

By leveraging you existing properties, you might be able to get a place for cash and start cash flowing that back into the LTR's to pay down the principle or work on a 2nd STR. 

You should spend some time to learn basic accounting principles. it will help hereIncome statement will show income and expensesbalance sheet will show assets and liabilitesthese are connected resources but provide quite different information.Assets and income are not the same at all, just as expenses and liabilities are not the same at all. 

"What if I gave you $40k down and $2k a month principle payments for x months"? 

If you subtract the principle and interest from the rent, is more than 50% of rent left (i.e. does it cash flow with the 50% rule)? 

Taking into consideration @Davidgreen book, Buying long-distance real estate investment. ( You kind of have to buy sight unseen) .Do you have a formula that will cover potential errors or unforeseen circumstances? 

Looking through my Audible playlist, these stick out to me as far as general economic principles for college aged students in no particular order.1.

Any Columbian on BP gets a vote on principle, @Greg Hills.

Saw that one come up yesterday and wish I had the funds to buy strictly on principle that sub $100k PPU is rare!