50% rule to quickly evaluate operating expense

[Marcello Di Gerlando]

I ‘m looking to dive into the multifamily market, I’ve been doing my home-work and I’m working with a realtor…I have lots of questions. Here is my first.
If I am using the 50% rule to quickly evaluate operating expense for a prospective multifamily property. Should I also add vacancy and credit loss to estimate Cash Flow before Taxes? Or does the 50% rule include vacancy and credit loss?