50% Rule- Is it Really a Rule?

[Emily Sulliban]

I've been reading on the 50% rule for calculating viability on a potential rental income property:
Gross monthly rent
-(minus)50% for operating expenses
______________________________(equals)
NOI(Net Operating Income)
-(minus)mortgage payment
______________________________(equals)
Cash flow (positive or negative)

My question is: Does the 50% rule ever become.. the 65% rule? Or is the 50% rule an actual rule and it's safe to assume that operating expenses will always stay at 50% or less?

Are there any landlords that can tell me all of their actual operating expenses (other than loan/mortgage payments)? I understand utilities, taxes, and landlord insurance.. the solid monthly fees that I know will come every month. The other fees, such as maintenance, legal fees, and eviction costs don't seem like solid monthly expenses... does the 50% rule account for these fees that will eventually pop up, but at an incalculable point in time?

Thanks,
Emily