CapEx reserves is the one I'd push people to actually track, since it's the easiest to skip when things are quiet. Cash flow and vacancy show up on their own, you feel them every month. Reserves don't complain until the year your roof, HVAC, and water heater all decide to retire within a few months of each other.
The number I watch closest isn't a flat percentage of rent or property value though, it's tied to how old each major system actually is relative to its expected lifespan. A property with a 3-year-old roof and a 3-year-old water heater needs a very different monthly set-aside than one where everything's pushing 18-20 years, even if the rent and the property value look identical on paper.
Curious how you're currently sizing your CapEx number, flat percentage, or something tied to actual system age?