Cash Flow / 50% rule

[John D.]

I just want to be sure I am clear about the 50% rule relating to rental properties.  If rent on a 2 unit is $975 and $895 ($1870 total)...50% of that is $935.  If P&I and taxes totals $941, this property is probably not going to cashflow correct?  I do realize the 50% rule is just an estimate and one could break down the expenses individually.  

I'm looking for some opinions on this rule and if anyone has had success using any different calculations.

Thanks everyone.  This is truly a helpful site and I started listening to the podcasts a few weeks ago and am up to show 32.  Very informative stuff.