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

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

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

@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.

Are these markets dried up until the rent to price formula evens out a bit more down the line (if they ever do)?

I do think the MTR/STR mix makes the most sense for Austin - MTR most of the year but switch to STR to maximize during the hottest travel months (spring for SXSW, fall for ACL/Formula One/UT Football)

I heard a guy on Bigger Pockets video give a formula to use to consider if you should sell: If your equity is more than 7 years of cash flow, then you may want to consider selling to get your equity out. 

. $550  / mth * 8 unit = $4,400 * 12 mths = $ 52,800 GRI  ****not $53,800****The formula for CAP = NOI/PriceNOI = [GRI - V&C loss + OI] - EXP  *****You did not subtract for expenses to arrive at NOI nor include a vacancy and collection factor****Assuming expenses are 50% and 100% occupancy (which 100% occ is not realistic all the time).

There is more to the formula on how much you can bring in, assuming you're self-employed.If you're an S-Corp, you can do the 23k as an employEE, then an additional 25% of your salary on top of that, with a maximum of $69k total. 

Bottom line: make sure you have an active, documented role in management, and you don’t get compensated using any formula that plugs in the amount of money that you raised.