If you are looking for how to define the format for percentage representation in the Excel writer, try set "0%" to the Custom Number Format field for the column.
(FME 2015.1)
Takashi
In FME 2015.0, select "Custom" (forgot correct choice string...) in the Format type, and then enter "0%" into the "Custom format string".
I'm not sure whether this works as expected in FME 2015.0. If possible, it's better to upgrade to FME 2015.1.1.
You can divide the value by 100 using ExpressionEvaluator or AttributeCreator, before writing.
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