Best Way To Calculate Landlord Fees and Determine True Cash Flow?

[Account Closed]

I am in the process of purchasing my first buy and hold investment property.  I have used the BiggerPockets analysis tools, which are helpful, but I'm confused on how to calculate true cost of owning a property.  I don't really know what the "50% rule" is actually covering (i.e. does it include mortgage and taxes, etc...?) and I'm not sure what costs I should be adding.

For example, suppose a property generates $3000 a month in rental income.   Tenants pay utilities.  Obviously I need to subtract mortgage, interest, taxes and insurance from the $3000, but how do I estimate additional costs that I will have to pay?  I can think of things like snow removal and occasional repairs, but is there a standard percentage that can estimate what that typically costs?  

Using my example, suppose I generate $3000 per month.  Mortgage P&I is $1800 per month and insurance is $100 and taxes are $300 per month.  Without considering anything else, cash flow is then $800 per month.  But what else should I add for my costs?  Perhaps assume maintenance costs are equal to (i.e. 20%?) of rental income?  Is there a formula?

Thanks.