How to plan a budget for condo reserves?

Our new 4-unit condo has been in operation for about 2 years under a property management company. Now we would like to self-manage. We have a bare-bones budget, but what suggested percentage should we add to the monthly dues so we have at least a minimum cushion to pay unexpected bills which always seem to surface along the way? How much cushion has generally worked for other very small self-managed HOAs?