@Lala Weiss - You will never get ALL investors to agree to the same metrics, because not all investors have the same goals. And your criteria/metrics should be tied to your personal goals in real estate, as well as the realities of the market in which you'll be buying. Which is why I started that very long post with suggesting you think about your goals and set some criteria that will help you achieve those goals.
I would say that it seems MOST content I've read suggests having at least $100 - $200 in cash flow per door, after paying the mortgage, management and all other expenses. I think it is smart to have a minimum cash flow goal, because anything less than $100 - $200 per door could easily be eaten away by higher-than-expected repairs or utility bills... and then you end up in the negative cash flow zone. Plus, real estate requires enough time and money that you probably shouldn't accept less than that per month for your efforts. I do think most investors have a cash flow requirement of some sort, but it can certainly vary. An investor who's buying properties with cash (and therefore doesn't have a mortgage payment) or is self-managing (and doesn't have the cost of a property manager) can and should expect more cash flow per door than someone like myself, who is leveraging my properties and hiring management to handle the day-to-day operation and tenant interaction.
I would also guess that most investors have a CoC Return criteria, because it's smart business to understand what kind of return you're getting on the hard-earned money you're investing. If you're only getting 4 or 5% CoC Return on the capital you're investing, you'd be better off investing in the stock market, which I believe has historically returned an average 9 - 9.5% over the past several decades. This is why I set my CoC Return at 10%, because I want to know I'm at or above the returns I could get by just throwing my money in an index fund and not having to worry about it. (Of course, there are other benefits to real estate besides just the CoC Return from your cash flows, such as tax benefits and appreciation, but I look at those as the cherry on top.) Again, other investors may set different goals... for instance, someone who has been in the game a while and gotten really good at finding amazing deals and managing their portfolio super efficiently might not accept lower than a 15 to 20% CoC Return.
When it comes to some of the "rules of thumb" like the 1% rule and the 50% expense rule, these are really more quick analysis tricks to help you understand whether a property is worth looking into further... not final criteria for whether or not you should buy. For instance, when I get an MLS listing that looks interesting, I do a quick online check to see what it would rent for and do the math to see if it meets the 1% rule. If it does, I'll take the extra few minutes look up the county records online, run numbers in more detail, and potentially schedule a viewing. If it doesn't meet the 1% rule, then I don't even bother (unless I think I can get it for less than list price). Furthermore, there are certain areas of my market where I know taxes are much higher and therefore even the 1% rule doesn't work... it usually has to be a 1.5% - 2% property to offset the high taxes and still cash flow. So get to know your market and then use these rules of thumb to focus your search, rather than using them as final criteria.
The bottom line is you have to know your goals... do you want a certain cash flow per month to live off or is there something else you're focused on achieving? Once you know your goals, think about the realities of your situation... do you have money to buy properties for all cash, or do you need to finance them? Do you have time to self-manage, or do you need to hire property management? What's possible in your specific market, based on average home prices and typical rents? All of these factors will help you understand what criteria are feasible while reaching your goals. My example... I want to build a buy-and-hold portfolio that generates a steady $10,000 per month in passive income for my family to live off. I don't have millions in cash lying around, so I need to finance my properties, and I work full-time so I want to hire property management. Both these things cut into potential cash flow, so I am happy to accept $100 - $200 per door in cash flow, so long as my CoC return is at least 10% (outperforming what I could do over the long run by passively throwing my money in the stock market). So assuming I average out at $150 monthly cash flow per door, I know I need about 67 units total to meet my passive cash flow goal.
Obviously, you need to put a lot of time into setting your goals, getting to know your market, and then crunching some numbers. If this is still confusing and you need a place to start, consider just borrowing my criteria to start running your numbers... 10% or greater CoC return, and at least $100/month cash flow per door (so $100 for a single family, $200 for a duplex, etc.). It's a place to start, and you can adjust as you learn.
