How do I know if the rent will actually cover my costs?

Covering your costs comes down to comparing real rent for that address against your full monthly carrying cost, not a headline average. Add the mortgage payment, property tax, insurance, any HOA dues, and a maintenance reserve, then set that total against rent for comparable nearby units. Brian runs those numbers against actual comparables for the specific property before a buyer commits, because a metro-wide average hides the difference between one street and the next.