Reading it again, it is kind of hard to understand
The people I know who professionally own investment properties own single family homes, duplexes or commerial properties. They are always willing to buy lower, but look for at a minimum the costs to be covered within 1-3 years 5 if things go bad and they all have extensive zoning experience. After 5 years they are expecting a very positive return and after 15 they are buying another property with the cash flow to lower their profit and tax burden and increase their equity. Those numbers can fluctuate depending on the activity and tax status of the rest of their portfolio as well. I am sure condos and apartments have even more variables. The two I have asked about it don't do residential with over 8 units because there is too much risk per property for small to medium sized investors, and they only buy projects outright, no individual condos.
There are so many variables it is really hard to pin down a perfect ratio, and the risks for flippers or small investors with under 5 properties and limited equity are a whole different set of numbers. Buy a condo or a property with a HOA and you have even less control.
I don't know any flippers, so when I ask questions of people I know the responses generally have to do with leverage cash flow taxes and equity, not a mortgage cost to rental income ratio.