What rule of thumb do YOU use to calculate maintenance expenses?

[Matt Lefebvre]

I've heard a lot of conflicting advice over the years on how much to "estimate" maintenance expenses for when doing a quick analysis of a deal. I've heard things ranging from "15% of gross rent" to "$2 per square foot to maintain in its current condition + $0.50 per square foot to be put in savings for a rainy day fund".

Obviously you need to go in more depth when seriously considering a deal... but what is the rule of thumb that YOU tend to follow when estimating maintenance expenses?