Rules of thumb are places to start - general guidelines. They are for initial planning.
One should dig past those when actually in the thick of it.
I would use a rule of thumb when making projections. I wouldn't use it on properties I already owned.
What you need to do is evaluate the property itself and figure when you will have maintenance costs. Project the roof replacement. How much life is left on the water heater?
Get the detailed expected depreciation and capital expenses for the individual properties in question and store that. Add a buffer for downturns, extra repairs (that tend to group together, rather than being nicely spaced out), extra vacancies, reduced rent, etc. Some of that will come down to what you are comfortable with, if the property has debt service or is free and clear, as well as your other sources of income that could potentially cover problems (are you still working, do you have a pension, do you have other income producing assets, how flexible is your spending, etc.)
That being said, if you just want a rule of thumb, here's the one I use (and yes, I realize by posting this everyone is going to ignore all of the above and just use the below, so you should be aware of that and purposefully ignore the below and do the above):
6 mo PITI + 20% of annual gross rent saved per property (for repairs, maintenance, etc.).
Again, this is so broad as to be almost meaningless (a property built in 2013 will probably need less maintenance than one built in 1972, some areas have higher vacancies, etc.), but there you go.