Newbie Question: How do you accurately forecast cash flow?

[Neil DiMuccio]

Hey there! I'm new to BP and new to REI in general. I'm still a renter and looking to first-time homebuy (or house hack) in the next 6-12 months.

I'm noticing that as I scan Zillow for properties, I'm unsure what I should be expecting for annual costs of a property (I say annual because thinking in terms of monthly it is easier for me to leave out some bigger occasional costs that may crop up). So far I've been thinking about Mortgage + HOA (if any) + occasional repairs.

I'm wondering - how can I get my cost (and income) numbers truly dialed in so I can have a very clear picture of potential cash flow? What kinds (categories) of costs do you forecast, and how do you get your numbers?

Thanks!