But I am seeking a mathematical solution

With that objective you can mathematically work your way back to what you consider a "reasonable offer"  i.e. how much Net income (Rent minus expenses) will the property produce versus how much cash you put in to the deal.  

You cannot simply use a mathematical formula as while one is simple, they will have to work with the engineers (civil, MEP) to coordinate exterior, entrances, grades etc.

Is this worth pursuing further, on paper- it does make mathematical sense.

It’s a mathematical formula based on number of people, square footage, etc.

Not arguing existence of climate change, that's a mathematical fact.

Mathematically, one option is slower growth (which you agree to).

I definitely wouldn't recommend this as you increase the risk of your HELOC lender eventually freezing your account, plus I can't see mathematically how you are saving extra money this route as the only savings that I can tell are from shielding the HELOC interest payments by keeping the balance at $0 (or lower than it naturally would be for those withdrawing more than their monthly expenses).

Everyone agrees (because it is mathematically correct) that to retire young the most important first step is a high savings rate. 

While there are differences when it comes to investing in Mexico, the mathematics are the same on both sides of the border!