It's like multiplying yourself 100 fold. 

And the criteria they use is self admittedly very focused ("They calculated the rental return for each county by multiplying the 2014 fair-market rent for a three-bedroom home by 12 (months) and then dividing that yearly total by median sales price for the county.")The other points that you touch on are most probably to be found on more specialized sites in reports that require the diligent investor to cast a wider net and delve deeper in order to filter out, absorb and deploy the underlying knowledge found in all those disparate sources.

Multiply by 60 if the park pays water + sewer, and by 70 if not.  

Multiply that x 40 doors and you are at $4000/mo.  

Steps, I know are: Start at correct ARV with a CMA, then multiply by 0.70% to hedge against the inevitable,  then continue to deduct ALL holding expenses for anticipated length of time for rehab.

Then I was told you multiply the total rent from the units by 12 and then multiply that number by 7.  

I try to market the end buyer price as a certain percentage CROI for flips or rent multiplier for buy/hold types.

More important, what is the average GRM (Gross Rent Multiplier) for the area?

The 2% rule is a form of Gross Rent Multiplier.

If you hire a giant multi-national consulting company, multiply those rates by three.I do not know the age, but in that area it is probably old enough to need asbestos and lead surveys, so that would be another $5,000 to $7,000 (depending if was built before or after 1960).