How do you calculate appropriate reserves?

[Emily Powell]

How do you calculate how much to keep in your real estate accounts for reserves?  Do you set aside a % of the monthly rent each month?  A flat amount per property?  Presumably this would differ depending on the age of the property(s), but I'm curious how people calculate how much to set aside for a rainy day/repairs/vacancies/etc.  Ultimately, I'm trying to figure out how much of my cash I'm going to use to purchase properties and how much should stay in the account as reserves.  Thanks in advance for your input.