rental house expenses

[Nigel Ford]

   Ive noticed not surprisingly that when I lower the expenses evaluating a potential rental house, its much easier to find a good deal that cashflows. In John Shaubs book "building wealth one house at a time", When he evaluates a potential rental house he only puts tax, insurance, and repairs/maintenance as a 20% of rent expense. When I lower my standards to these expenses it seams a lot easier to make a deal, but at the same time i want to be safe. What do you guys think is that enough? 

Nigel