I believe, off the top of my head, it's single digits of rents actually have rent reported to bureaus. 

You are correct, that this has long been the conventional figure used to estimate cost for buildable sf in NYC. 

It’s given me the opportunity to save and prepare for my next upcoming purchase.I’ve gotten the units from $750x2 and $800x2 to a consistent $900. current mortgage is $2500 monthly, refinancing to a conventional and removing mortgage insurance should bring me to $2100. 

Doing a conventional loan for each property is out of the question due to my debt-to-income ratio. 

If you can find smaller local banks in the market you pick they may have more unique products that they keep on their balance sheet or try and do the 5% down conventional on a 4-unit  FHA's self-sufficiency test applies to both 3 and 4-unit properties.

9) adding multiple ADUs or adding an ADU to a quad looses F/F conventional financing.

I cannot qualify for traditional conventional for multiple reasons but the over 200 to 1 debt to income ratio is a primary reason.  

Conventional loans jumped to 12 months seasoning about a year ago.

This is based off of the conventional loan limit where you are buying.

@Mike Klarman have you had issues with getting a conventional loan under an LLC?