@Michael Plante my tenants pay monthly (although I have heard of others in various markets charging by the semester or year)

What formula or criteria do you use to decide whether to flip or to hold?

The areas you specifically mentioned have seen growth due to those factors (Buckeye - industrial expansion; Surprise - TSMC plant relocated). 

It's a smaller town that is seeing growth from a billiion dollar EV Battery plant that Honda is building about 15 minutes away from it. 

For the purposes of property tax, they change formulas starting 4+ units to commercial models. 

@Tim Delaney you asked what the return on equity would be and I'm not sure--I don't have that formula in any of the books I've purchased.

As long as you are coming out of pocket at around the same you would be renting somewhere else then you are still getting ahead in life because your tenants are helping pay down the loan and build your equity.You should also run an analysis putting in what the market rate rent would be of you did not live there.

I haven't run any numbers on this, but I've seen similar setups and at some point you are really just ahead the month. 

Investors base their purchases on potential, but appraisers base their numbers purely on formulas and comps.

They saw ahead that these jobs take a toll on you so bought RE and now live off RE in one form or another.