Second, your supposed private lender isn’t even good at math.

Specifically commercial multifamily, but also interested in finding duplex, tri, quad opportunities if the math makes sense.

Only you can do the math and see if that makes sense. 

Now if you're clever at math you see those numbers only equal 37-40%, but I include a "Go to #ell" budget of 10%, which is what I fall back on when everything blows up at once and/or just nickles and dimes me to death.I know there are many folks who swear they can make money on SFHs/duplexes with 1% of "all in" cost to rent ratios and 25% expenses budget. 

How do you do the math to determine total for ARV?

On current income back of the napkin math puts you at under a 5%cap which is trash.

I’m currently a middle school math teacher so I have a strong math background, but no “REI related” experience.

Depends on what's customary in your area.My back-of-the-envelope math shows ~$550/month cash flow and a CoC ROI of 17.5%.

I would add 1) read Brandon Turner's book on Rental Property Investing, he consolidated a lot of great information into one place that answers your four questions above and 2) choose which type of property you want to focus on and 3) start analyzing properties (do the math) for cash flow (if you are buy and hold), analyze at least 40-50 properties before you make an offer.

Understand the basic math to reach profitability whether its a flip, cash flow etc..