Nothing seems to pass the 50% rule

[Jose Alejandro Hernandez]

Just starting out here and been reading Brandon Turner’s book on Rental Property Investing.  The book gives a rule to use to quickly screen properties known as the 50% rule [(0.5*projected income/rent)- (Mortgage P and I)] to determine projected cash flow. I have been looking through properties in El Paso, Tx and even other areas on Zillow and applying this rule. Everything seems to come out negative! 

I have been calculating using Zillow estimates. Are these estimates good projections? Any tips to find better deals for a beginner?