Percentage Fee Option Under Student Extra Fee Settings

We would like to be able to add a percentage fee that would be specific to that student. Currently, under the Extra Fee option or Tuition option in the student settings, we can only specify a dollar amount for an extra fee or a specific tuition amount.

We are in AZ and frequently have ESA students which we add a 10% fee to their tuition/classes to cover the ESA servicing fees we have to pay