When creating a formula that should return a percentage value, you will need to format the field correctly to get the "%" sign to properly show.
Right-click on the field in question, select Format Field.
Click the "Display Currency Symbol" checkbox.
Click the "Customize" button.
Click the "Currency Symbol" tab.
Change your currency symbol from "$" to "%".
Then change the "Position" to "-123%".
This forces the percentage symbol to appear at the end of the numerical value.
This will give you percentages thus: "87.22%".