Under the Validate tab you can set the range allowed for that field. If the
field's Format is Percentage, use 0 to 1. If it's just a numeric field you
can use 0 to 100.
I would create the text field with the "Percent" format. For the validation tab use the value between 0 and 1. The values will be entered as decimal number or for 15% one enters 0.15 for the value and the display when leaving the field will show 15%. Also the value of the field can be used in any calculation without any adjustment to make the "percentage" the decimal number needed int he calculation.