@Jonathan Brown, what you should do is make a short list of all the major systems that fail over time: AC, Water Heater, Roof, etc. along with costs to replace each one. You can estimate these, or even get quotes if you're not sure. Then divide by their expected life in months, which again you can estimate or look up. This gives you the amount you need to save each month for each of these major systems.
But of course, that only works if the systems are new when you get them. if the roof caves in tomorrow, you need to be ready for that. So you also want to have and idea of the rough life remaining in each of those, which again you can estimate or ask an inspector or contractor. Then you just have to make sure you set aside enough extra to pay for them when they actually fail.
Lots of guesswork in their of course, but this is how you should be thinking about it. And those monthly numbers are important when considering your real RoR.
Example of roof only:
- It costs 18k to replace.
- it will last 360 months.
- It has a life remaining of 180 months
So it's going to cost me 18k/360 = $50/mo to save up for replacing the roof. But it's already halfway used up (180/360), which means I either need to start with 9k for it and save $50/mo, or I need to save up at twice the rate (100/mo) over the next 15 years.