Mathematically speaking don't pay quarterly taxes if you don't have to, but be sure you have enough to pay your taxes when they come due.

I love learning and during my spare time, I am always reading books about personal development, software engineering, mathematics, physics, and being a successful introvert entrepreneur(Just call me Elon Musk Jr lol). 

Realize all of the above are not mathematical, so its your "assessment".Then, what's the answer.Lets say your future payoff is $200,000 in 15 years. 

A 9% cap = $577,777 (mathematically divide the NOI by .1 for 10% or .09 for 9%, etc). 

She was trapped by debt once, understands through Dave Ramsey that getting out from under it is as much about psychology as it is about mathematics.

Constructing the additional unit will mathematically generate more income in the long run, but I am concerned the smaller bedrooms could be a negative when marketing as luxury. 

This is where you determine if it is SF or MF as the vehicle that will get you to the strategy to achieve your goals.Both SF and MF have pluses and minuses, you have to make sure that is part of your strategy to take you to your goal.The reality is that when you own real estate you own a business, that business has processes, procedures and structure.Part of that are systems wrapped around those tactics so that your are not dealing with the actual property if you are truly running it like a business it wont matter because owning real estate is a purely mathematical equation based on the numbers not on emotions.Just my opinion for what its worth.

And it was all 100% predictable and mathematical, and completely ignored.  

I'm looking for a financial/mathematical approach to making these decisions.

Aside from all the logistical and mathematical issues with supply/demand cycle of all this, there is now a new component, PROFIT.