Great question @Mindy Jensen! While I am a relative newbie, I am a nerd and a numbers guy and this is a topic that always catches my attention.
When I analyze my rental properties (using the BiggerPockets Rental Property Calculator, obviously) I approach it much like @Scott Trench. I will add in $10K of additional rehab dollars to fund a CapEx/Repairs account. I will run my numbers with and without this allocation, but including this will result in the most conservative approach. I will then add 15% of monthly rent into this account. If I don't end up needing the initial $10K, all the better. Of course, with my first property, I needed a hot water tank within the first 4 months and I had the funds set aside to cover this with no issue.
The part of this discussion that frustrates me is how many investors throw a % number blindly at things like CapEx, Repairs, Vacancy, etc. To get an accurate sense of the numbers, there needs to be a little more thought. 15% of the total rent I receive for my triplex in New Hampshire is very different than 15% of the rent for a triplex in Boston, or Wyoming, or Colorado. However, a hot water heater costs what a hot water heater costs. Sure repair costs will vary some based on location, but these rule of thumb percentages do not apply evenly across the board. My 15% may be enough to cover repairs and CapEx, but it may not be enough for someone else. For a quick view of a property, I aim to be conservative. 15% total for CapEx/Repairs, 8.5% for vacancy (1 month vacant). If the numbers work out using these assumptions, I can move forward with a relative amount of confidence.
I am looking forward to reading the other posts on this thread...