HUD uses data from rent surveys for 2 bedroom units, supplemented by general rent trends in smaller areas that may not generate enough data in rent surveys on an annual basis, to calculate the amount of rent paid.

Just some of it may not be able to be on the contract itself and instead on a supplement or a bill of sale.

Rules of thumb are a very lazy way to do business.You can build a generic pro forma and plug every applicable number in, take five minutes, and come up with a reliable look at your future CoC, CoC including principal repayment, IRR, etc.Build your pro forma to reflect the base case and then the pro forma case.

Basically supplements social security.

Ex: Cashflow $500-900 a month (depending on financing) and use that to help supplement part of a $800-1200 mortgage which can buy you a lot of house in some nicer parts of Delco. 

Our goal has been to build cash flow to supplement an income of $3000/mo.

In the supplemental material provided when buying @David Greene's book "Buy, Rehab, Rent, Refinance, Repeat The BRRRR Rental Property Investment Strategy Made Simple" (which I should have in the mail soon), he mentions an operating agreement with your private investors in which their is language that promises them first lien or position.

I know that sounds generic, but each site has its own challenges and costs associated. 

Having said that, some lenders will allow you to do a supplemental loan on top of assuming the $1MM loan in the above example so you might be able to get back to a 25-30% down deal just like getting a whole new loan.

You can also use BiggerPockets as a supplement.