I started tracking every system's age vs. expected lifespan

For years I budgeted maintenance the same way most people do, a flat percentage of rent set aside "just in case." It felt responsible. It wasn't actually based on anything.

Last year I went through every unit and logged what I had: water heater install date, HVAC age, roof age, appliance ages, and lined them up against typical lifespans for each. Nothing fancy, just a spreadsheet.

What changed wasn't the total amount I was saving, it was the confidence in the number. I stopped being surprised. I knew the water heater in Unit 3 was on borrowed time two years before it actually failed, so when it did, it was a line item, not an emergency. I also realized I was over saving for the roof and under saving for HVAC, which a flat percentage would never have told me.

The uncomfortable part was realizing how much of my past "maintenance budget" was really just a guess dressed up as a plan. anyone else doing this at the individual system level, or is a blended percentage still working fine for your portfolio size? Genuinely curious where this stops being worth the effort.