I have a ToolTip with a value set as:
Value=\"{Binding Path=DataItem.EquitySold, StringFormat=Reserved (Equity Share: \\{0\\}%)}\"
The tool
You don't need to escape the brackets. Try this (i like to put the format in single quotes):
Value="{Binding Path=DataItem.EquitySold, StringFormat='Reserved (Equity Share: {0}%)'}"