A % is actually a variation of a currency format. To Change a currency format, right click the object, format, number, customize, currency symbol.
Make the currency symbol a trailing rather than leading symbol, and blank out the $ from the currency symbol field, as we are going to enter a formula to determine the currency sysbol.
Conditional Format the currency symbol by clicking on the X-2 field to the right of teh currency symbol field, an enter the formula:
if {fieldname}=0 then "" else "%".
You're done!
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