What % Cash Flow Should I Consider Reasonable?

[Casey Rondinella]

I commonly see people mention cashflow in dollar amount i.e $500 a month as a measure of how successful the property is as a producer. 

What I am looking for, is more clarification on what % cash flow relative to total purchase I should be after. Or, is there a more intelligent number should I tie this to? 

In summary, I am looking for a good cashflow % formula I can use to calculate if a properties cashflow gives me enough margin of safety + ROI in order to more properly calculate if the property is a dog or not