Of course it's personable and will depend on an individual's financial situation and "how much I want to deal with b.s. factor"  I was looking for some general rule of thumb like "never take less than 80% of fair market value" like 20% down is a general rule of thumb for purchasing a property through conventional loans.

Biggest factor is determining market value for each house and the market rents. 

I'll probably make it into an efficiency apartment, someday.  2 top floor apartments have their own electrical meters, 2 bottom floor apartments and garage share the one other electrical meter.  

I will look into my AGI and see where I land in that category, as for what I see right now that will be the determining factor.

I worked as a mortgage broker for years, but I was hoping there is a more efficient way other than waiting 30 plus days and jumping through the hoops.

For example, if a major corporation is opening a factory within the next year, that will create 1,000 jobs over a 3 year period, is that a good incentive to potentially invest in that city?

You get efficiencies of scale and can build faster.Initially, you may not make more cash flow from 4 leveraged properties than you do from one free-and-clear property, but your getting 4X the appreciation and depreciation.

And, its a lot more efficient.

If it is just an area with expected higher cash flow due to nature of tenants and lower quality area then no I won't touch it.The other factor is how much time this might take away for that expected 6,000 gross cash flow versus your other business practices.  

Without all three knowledge bases, working together as equal parts of a whole, you can reach success quickly and efficiently.