This is not possible within the RPD, as percentage is not a datatype. However, when you override the column data format, use the "Save as Default" drop down to save the format as a system-wide default for the column.
As suggested above, adding the deciimal places in the formula is a neat trick to force the percentage to show decimal places properly. Not very good functionality I know, but the solution suggested works around this shortcoming nicely. I recall the same applied to OBIEE 10g.