How much money should I save per property in a slush fund?

[Sean Smyth]

I'm finally at the point where I'm researching deals and am reaching out to team members in some areas in the Midwest to start finding me deals.  However, I'm not really sure how much money I should set aside for repairs and vacancies.  I've talked to a couple investors who had wildly different approaches.  One person tells me that they save HALF of all income per property in a slush fund in case something happens, another person I spoke to only has $10k allocated per property.  Is there a general rule of thumb that will keep me out of trouble?