FINAL DISCOUNT changes (in Business Hub)

When you use Final Discount % in Business Hub and save the order and then go back an make a change to the order that changes the total the Final Discount % changes and doesn't stay constant.  (i.e.  If I want to give a 10% Final Discount to a customer and then change something on  the order that changes the total the discount will either change up % or down % in stead of staying the constant of 10%. 

This should change to be a fixed % and change with any changes to the order.