Costs to rent received ratio?

[Mark Forest]

I am going to have to review the book What every REI needs to know about cash flow, but for now I would like to know what ratio you all use to determine costs verses potential rent received. As you work on a house and you see the expenses going up and up at what point do you say this is not going to work? What is the correct ratio and what numbers do you all accept? Thank you in advance