Hi,
I'm pretty new at this real estate stuff, so please take my answer with a grain of salt, but this is how I did it.
I used the four square calculator on bigger pockets. That usually only calculates cash on cash return, but I added in other things as sources of income, even though they weren't technically monthly income. I added in appreciation and mortgage paydown. That gave me a pretty good sense of what the overall picture was, not just the cash flow.
For example, my current rental has a slightly negative cash flow after all expenses are accounted for. However, because of where I'm at in the amortization schedule of the loan, I am paying off about 900$ per month in principle.
Also, in the years that I lived in it(recently just turned it into a rental), it appreciated about 6,000 a year, and would probably accelerate from there, since the area is improving.
So, I looked at the 500/month in appreciation and 900/month in principal paydown, and not just at the fact that it had a monthly negative cash flow. Factoring everything in, the picture us a lot different than just the monthly income.
Hope that helps!