I am not sure I understand what you are saying here. If you changed the format of a number to a percent, it should show a percent sign (and you did not post one in your explanation above).I was thinking the same as you in just change the formatting. In testing to make sure that it worked I entered 90.7 into a cell that had General Formatting. I changed the formatting to Percentage and it change to what jimboayr got, 90.970
=(C4-D4)/(C4)
Why are you multiplying by 100? That is why you are off by a power of 100!
Also, you do not need the SUM function. SUM is used to sum up a range of cells. You are just doing simple subtraction.
Just use:
Code:=(C4-D4)/(C4)