Before buying, estimating cash flow & expenses on MF properties

[Nicholas Judd]

Hello BP, it's my first post so go easy on me. 

How do some of you estimate what a property will cash flow and what your expenses will be before you buy? Is there a formula or perhaps a "rule of thumb"? 

I'm currently active duty and wanting to use my VA benefits to purchase a four plex but before going into it, I want to understand how some of you get your numbers. I don't want to be naive and just calculate rent and mortgage. What are expenses that are often over looked?