Much like anything in real estate the numbers depend on how you purchase it (i.e. with using it for yourself as primary driver, or making money as a primary driver).If you’re looking to supplement the cost of ownership, but are buying for yourself maybe in all reality covering the mortgage is good enough for you.

I am also OK with leaving some money in each of the properties as my income will supplement that. 

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.

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.

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.