Cash flow as percentage of expenses

[Christopher Za]

Pretty simple question, that my searches haven't turned up.

I understand cash flow, expenses, and the 50% rule. I also understand that people tend to look for a certain number for Cash on Cash return (e.g. 6-10%). What I can't seem to find is whether or not anyone every looks at just how much cash flow is enough given expenses.  

I'm in a situation where I am going to be putting very little of my own money into one or more properties (all of which are older construction). I've tallied the actual expenses and added my own estimates and the numbers happen to fall around 50%. However, the property only barely cash flows. If the unexpected big expenses arises, it could really hurt (I've budged 2% of purchase price for Maintenance, which totals $12,000/year for one property). 

So the question is this.  Do any of you have a guage that compares cash flow to expenses?   What do you consider safe? 10%? 20%?