Restrict the amount change percent on purchase order to only some extent irrespective of COs submit.

How to restrict the amount change percent on purchase order to only for some extent irrespective of number of change orders submitted by the user.

For example if we set an approval rule that if change order percent more than 5% then it should reject the order, then user can be able to amend the amount to only 5% irrespective of how many change orders submitted.(but not every time change order is submitted, if 5% is already amended in change order 1 then user shouldn't be allowed to change the amount to 5% once again).