You could use a expression on the format of the textbox. YOu'll need to likely tweak this a bit, maybe use a Row Number function instead of the ID's value, however something like:
=Iif(Fields!ID.Value = 1, "0.0%", "0.0")
Note, that you need to ensure that your values are coming through correctly for percentages. So, without the percentage, your first row will have a have of 0.6 not 60.0 (as 60.0 is 6000%). if this isn't the case, you need to also manipulate the value of that row, using a similar expression to above.