If the FHA loan route doesn't allow that, you could look into some local lenders to get to the structure that you desire.   

The right corner of this property's basement is in a flood zone, which makes this house less desirable to some.

Realistically, everyone has a different desired profit (vs capital utilized) for there to be any generic or widespread rule-of-thumb.

If you are in a desirable area you might just want to think about converting the whole place to an Airbnb and run it yourself. 

I doubt it makes any difference in the assessed value of the home, for equity / financing, but it does make the home more desirable to tenants and buyers especially IF you are in a B market or above, and it is done tastefully (no tiger stripes). 

Less desirable locations need to offer free rent, TI money etc.If it’s vacant there’s a reason so you need to understand that reason and mitigate it.

Whether well and septic properties are less desirable in the market depends on the area.

Oddly, for flips, the calculator doesn't have a purchase price in there, just runs the numbers for your desired outcome and gives you a max purchase price to achieve your results. 

Is it that your girlfriend has no desire to get ahead in life, or is she just afraid of risk or the concept of flipping? 

I have never owned an income property before however I have found a multi family home in a desired area that I’d like to love in for a while.