Welcome Mark!
I know there are a lot of calculators out there. I use a very simple formula to START, but need to look at all factors.
Annual gross rent and cut it in half. So if you are getting 1k per month that is 12k per year and you will have 6k left that is your Net Operating Income. You will take your principal and interest payment out of that 6k and whatever is left becomes your cashflow.
Now the variables.
What class neighborhood is it in? Better areas typically perform better .
How old are the major items. Furnace, electrical, plumbing, roof, etc.. this could cause ya to be heavy on CAP X.
How many units are in the building? If you have a 12 unit for instance, you may need to consider, grass cutting, snow plowing, cleaning hallways and common areas bigger water/sewer bills and dumpster fees. this could cost you MORE than 50% of your gross rent in expenses.
Who is paying utilities? who pays ALL utilities?
Biggest thing to remember is that Sellers LIE. Read between the lines when buying a place. Ask to see tax returns and NEVER buy based on how a building COULD perform. Buy based on reality.
Sorry, I could go on all day about this, I tried to keep it short and to the point.
Good luck!!!